I believe the game is trying the model the fact that AP rounds rely on their kinetic energy to do maximum damage while the HE will be effective at it's maximum range because of it's warhead. Surprisingly, both types of ammo have a muzzle velocity of 1000m/s.
I have tested this out in the game, firing at a Bradley at 3km+ with AP rounds did very little damage (took over 50 rounds to kill it). Tried hitting them at inside 2km and 5-6 direct hits was enough to cause secondaries.
Just doing some research on the various 30mm guns, seems the 2A42 fires 30x165mm rounds which are nearly the same length as the A-10's ammo! (30x173mm).
The Apache's M230 fire's a much shorter 30x113mm round which is the reason for it's relatively low 808m/s muzzle velocity.
As for the 30mm gun's effectiveness on MBT's, it's just not worth the hassle in my opinion.