Got to love how the official manual that came with the tank contradicts another official source by a large degree. At any rate does anyone know what they mean by basis for armor? planned? and not what they ended up with? Or is it perhaps because of the hull being cast armor and the uncertainty of what they would end up with so they give a range?
Basis for armor would probably mean the armor that was planned on the blueprint, versus what was produced in real life. Common problem when working with rolled steels really, as rarely do you get it to the planned thickness. In fact, more often than not the thickness is like plywood in a way. Meaning that it's purposefully thinner. For example: 1 inch thick plywood is actually just under an inch (usually a quarter inch under thickness.)