The Tiger II gets +40 penetration from far to close range too. Is that a flanking tank too then?
There nothing magical about the number 40 that makes a unit a "flanking" unit.
You should also be aware by now that absolute values mean little. That is why weapon profile curves are used.
The T-34/76 get a x150% penetration increase while the Tiger gets x120% and thus the T-34/76 has more reason to close in to its target.
This is an intentional decision. It even explained in Patch notes about the PzIV buff:
"Panzer IV
The Panzer IV is having its ranged penetration increased to better combat medium tanks at a distance where it has the advantage of stronger frontal armour and faster rate of fire.
Penetration from 120/110/100 to 125/115/110"
The PzIV has it penetration increased by 10 far but only half that to mid and close so that it benefits less from closing. The player is encouraged to keep the distance when using the PzIV.
How is this an assumption? You can check the values yourself. The vast majority of vehicles get a steady +/-10, +/-15 or +/-20 penetration per subsequent distance.
You are looking for pattern that is simply not there, specially since the penetration values have changed with paches:
Panzer IV: +5+10
Cromwell: +15+15
Sherman: +20+20
T-34/76: +20+20
There clearly is no formula for these values, they are simple fixed increases. The Panzer IV is the exception here (+5 and +10), not the T-34/76.
There is no formula but there is design. There tanks that want to close the distance and there tanks that want to keep their distance.
Since it seem you do not want to take my word for here it from Relic in black and white from the patch notes:
"The role of the T34 is that of a
flanker meant to exploit enemy weak points with hit and run tactics. "