It's an issue with call in meta because the IS2 has the same armor with better speed than the KT, and doesn't have very much problem penning the KT either so when your opponent can get far more IS2's than you get KT's it's a big problem.
the VI B having more armour then the IS-2 wouldn't change the fact that 2 IS-2s beat a single VI B as the amount of armour each tank has does not affect the advantage callins have. same with speed. it's also not a good justification to buff the VI B.
The PIV has a fairly pathetic chance to pen the IS2, it's just worse in basically aspect, while the IS2 and KT are somewhat equal in some respects.
all mediums suck against all heavies (KVs are debatable as they are "sort of heavy")