This is true in terms of the capability of the Sherman to defeat heavier armor. The kinetic energy penetrator was at the time the main way to defeat armor. The 76mm gun had a longer barrel (52 calibers, up from 40), which allows a higher muzzle velocity, increasing the kinetic energy. The 76mm ammunition was physically longer, with most of this elongation devoted to additional propellant, resulting in a higher muzzle velocity, increasing the kinetic energy.

In short, the extra 1mm is not the reason the guns are different. All other things being equal, the performance difference between a 76mm and 75mm gun would be basically unnoticeable. Changing the caliber didn't accomplish anything other than making the two guns easier to distinguish in written form, and increasing idiot-proofing.

Also - the "76mm" gun is not actually a 76mm gun, but rather a true 3-incher.