Thats fine, if German tanks with bigger or equal caliber guns don't get out-ranged by Sovs just because and not because it made sense.
Damn, I can't think of any German unit that might do a similar thing to the obviously implied SU-85... Oh wait, there's the StuG... Panther... oh and the uncounterable Elefant. Remember, the SU-85 is a tank destroyer with a hullmounted gun and terrible manoeuvrability. Range is the only thing it has. Sure, it out ranges 2 of the tanks I mentioned, but don't tell me Soviets aren't in a similar position with all their turreted tanks? (none of which have any range bonuses (as opposed to the Panther)).
Anyway, what larger calibre guns? The largest tank gun the Germans have is the 88mm. (the Brummbar is an exception but that short barrelled howitzer gun that it mounts is hardly an equivalent to the ML-20 on the ISU.) The gun on the SU-85 is (obviously) 85mm, that's hardly much of a difference. Meanwhile the Panther mounts a gun of inferior calibre to the 85mm and still outranges the T-34-85, what gives.
Anyway, Calibre isn't the primary factor in determining muzzle velocity and range. It's projectile weight, size and the length of the barrel. An APDS shot is effectively a 2 pounder shell (think something smaller than a T-70's shot) which is made to take the bore of a larger gun whilst still being much lighter, once fired, the things that make it fit the calibre of the gun drop away (hence discarding sabot) and the additional speed of the lighter shell makes it penetrate better. Calibre only comes into effective once the shell hits the tank. A 152mm shell from the ISU doesn't need to penetrate as the effect of the explosion would kill everyone inside the tank from concussion or metal splinters. Meanwhile the 2pndr shot from a tetrarch in vCoH would need to hit something pretty important to do anything, if it penetrated (this was improved by the little-john adapter)