Ofcourse I'm not excluding bounces from a ZiS, but I know for 100% sure, that when you shoot 100 shots on a Tiger with a ZiS and 100 shots on a IS-2 with a Pak, you will get more hits on the Tiger than a IS-2. Even when you consider RNG it will,
Today I learned that math is hard.
Is-2 Frontal Armor: 375
PaK-40 penetration: 210 / 200 / 190
Chance to penetrate at all ranges: 56% / 53% / 50.67%
Tiger Frontal Armor: 300
ZiS penetration: 200 / 190 / 180
Chance to penetrate at all ranges: 66.67% / 63.33% / 60%.
So no. The ZiS has a ~10% higher chance to penetrate the Tiger than the PaK-40 has to penetrate the Is-2. It's not complicated.