CHANGE IN 0.7.2
Voice messages were changed in the patch.
Instead of stating "Critical Hit!" or its variants, your commander now says "Penetration!" for any shots that only hit the tracks.
Otherwise, the mechanics below still apply.
Differentiation of Issues
The true zero damage hit/criticals is when a tank gets shot, say in the lower hull or an IS-4's driver's viewport, leaves a mark, but there's no damage. On the receiving end, the tank takes no damage, but lost a crew or module. An occurrence like this happened to me once in a blue moon. Out of my 12k games, I can count on both hands how many times it happened to me. We only remember it to be common because this issue is lumped with the problem below. Of course, this dilemma still a very important, but the solution to it may be more difficult to come by. This is a game mechanics issue, but Overlord said they are overhauling it in a previous Q&A.
The issue most people are talking about is rather a voice response error. The commander used to tell you hitting only the tracks is a critical hit, even if you hit the reduced damage area, and didn't penetrate the hull. A few patches before, the tracking mechanic was changed to the present. Before hand, hitting a track anywhere will generally destroy it, immobilizing the tank. From the 0.7.2 patch, the voice message was changed by the developers as a fix. Instead of "Critical Hit!", the commander now respond with "Penetration!". The commander says that regardless of whether the shell really penetrated the tank's hull or not. Needless to say this solution didn't work.
What doesn't help is the fact of shell dispersion when a shot is made. A shell has a 97% chance of hitting anywhere in the circle, so despite you may have aimed above where you pointed at, the RNG may made the shell go lower than expected.
Furthermore, roadwheels on tanks actually act as a separate layer of armour. Roadwheel armour ranges from a measly 5mm to an additional 50mm+ on the E-100. Since the roadwheels act as a separate layer of armour, shells may change directions or lose normalization after hitting the roadwheels.
Now what I'm betting is this is simply a localization error. I have contacted a NA community manager. He is passing my proposed fix up to the people at Minsk HQ.
My fix simply turns the commander's response into how spaced armour is currently handled. Spaced armour has two parts, outer and inner. If the outer spaced armour is penetrated, it will leave a gaping hole in the tank. However, if the inner armour is not penetrated, the commander will state "Didn't penetrate" or "Bounced". The message depends on how the shell turned out inside the spaced armour.
With my fix, if a shot hits the tracks, but did not penetrate the side hull armour, the commander will say it didn't penetrate. As opposed to the current "Penetration" for every hit, this will reduce the confusion among the player base.
Zero damage AP hit.
Red: Hull clearance above ground.
Yellow: Area where hits count as hull hit
Blue: Area where hits count as hull hit, as well as tracking the tank (If projectile damage is greater than track module health).
Green: Area where hits do reduced damage to track (1/3), but no hull damage. Will be explained.
White: Area where hits do not count as hull hit, but will track the tank (If projectile damage is greater than track module health).
As it can be seen above, all three have areas where hits do absolutely nothing to vehicle health (not to track modules however). Now, you may wonder why tanks have areas where no damage hull occur. That is a good question. The images below will answer that.
Remember the red lines? They're back. Look underneath the red line. What do you see? If you cannot see anything, you are correct. There is nothing there. This is why shots can hit the tank in the hull and do no damage whatsoever. T-54 has this "problem" because its hull lies above the ground way more than most other vehicles in the game. They also have low height, forcing tanks to depress their gun more, which coincidentally makes the gun's line of sight going directly into the ground.
Another explanation of course is the fact the gun's aiming reticule only means that your shot may land in there. Therefore, it is very likely your shot strayed from your aiming point, striking the green area.
This is likely the most common occurrence regarding zero damage criticals.
Zero damage hit on a hatch or viewport: Most vehicles have viewports weaknesses, but some tanks do not. Vehicles like the Maus have viewports hitboxes away from the hull. Happens very infrequently.
Zero damage HE hit:
A. Shell hit the track. If the shell was small enough calibre, splash may not be enough to damage the tank. (Armour absorbs HE damage to some extent) For artillery: Some shots impacting at very high angles reduces splash by quite a bit. You can usually see it in overhead when you miss. A normal splash explosion is very large, but a reduced splash from angle results in a small puff of dirt. When the angled hit impacts the tank, your splash is still reduced. Common.
B. Shell hit the turret. Shell most likely hit the gun, which is quite far away from the frontal turret armour. Thus splash damage may not be enough to hurt the tank. Common.
C. Shell hit the mantlet. Quite a few tanks like the Maus, Pershing, Patton, etc. have non-penetrable mantlets. AP, even HE shells are absorbed by it. Uncommon.
D. Shell hit T1 Heavy, M6, M6A2E1, T95, Maus, E-100, GW Type E. These tanks have skirt type armour near their suspension/tracks. HE shells are absorbed by the skirt armour. Common.
Zero damage hit without any critical hit sounds, leaves damage mark, etc.
A. Shot impacted a no damage hitbox on the tank. Maus has a big one in front of the turret, the little angled plate on top of the engine deck.
B. Shot hit skirt armour, but did not damage suspension. Side skirts adds additional armour for the shell to penetrate before doing damage. After hitting the side skirt, shell's penetration drop significantly. Tanks like the E-100, T95, are affected. IS-3/704/IS-7 are affected as well. The side armour of those three have two parts. 1 is a 30mm side plate at 45 degree angle, then inside it, there's a 90mm plate at 60 degrees. The armour there is ~200 without normalization. Below is an image of an IS-7, which shares similar side armour characteristics of the IS-3. Before 7.0, the IS-3 only had a 90mm plate at 45 degrees. IS-7 has a similar armour hit box to the IS-3, but the inner side armour is 100mm.
Shot B penetrated through both tracks, but hit nowhere near the hull. Considered a false penetrating hit.
Shot X ricocheted after striking the falseboard on the side of the IS-3, impacting the 90mm @ 60 degrees upper side hull at a high angle, or the shot did not have enough penetration to make it through both the falseboard/side hull. Shots like these get no voice report from your commander, but leaves a hole on the tank's armour.
Shot Y went through the track, and hit the lower side hull, where it's 90mm @ 0 degrees, damaging the tank. Considered a real penetrating hit.
IS-3 and 704 both have 90mm @ 60 degrees side hulls+30mm @ 45 degrees falseboard, IS-7 has 100mm @ 60 side hull+30mm @ 0 degrees falseboard.