Russian SABOT amo is lighter in weight than NATO ammo. but Russian tank guns and ammos have a higher muzzle velocities. however, the Russian SABOT projectile looses speed faster than western projectiles, giving it less range (for damaging effect on impact), and less precision. that's why at real long ranges they prefer to fire their laser-guided mini-missiles instead of SABOT, doing that via the gun tube (max 5000m
) However, at short ranges the 125 mm and standard ammunition is as lethal as the Rheinmetall 120mm and standard ammunition, for the kinetic energy is in the overkill range anyway. I would assume that a T72 should have not much problem to kill a T55 at 2000m. So it is probably the game, not your perception.
However, a lot depends on the SABOT type. the differences in penetration probability are huge, sometimes.
NATO tanks would prefer to evade fights at the long range of the T72//80, where the Russian tanks would fire at them (precisely!) while still being out of range for NATO ammunition, and evading the short range as well where even western top tanks would be highly vulnerable to the slightly inferior Russian SABOTs. NATO tanks therefore would try to keep the battle in the medium range, becasue that is where they would dominate against russian tanks.
ATGMs not taken into account, of course.