The explanation I hear most often is, the round went clean through, without exploding
Which can happen. If the AP round hits the target in an area that is not critical then it can mean no "critical" damage was done.
Also... FYI for everyone... I STRONGLY suggest to not ever take %100 AP ammo for your tanks, ever. The next time you see that M3, M8, M18, jeep, or other such soft skinned gv's fire an HE round instead of an AP round. With en HE round you do not need to have a direct hit to damage it or even destroy it. This is especially true when they are moving to a flank. A near miss with en HE round will disable *something* and render it a non-threat. I bet since the M18 has hit AH, I bet %75 of the ones I've destroyed have been with HE rounds. Trust me. Use HE against the lighter skinned gvs', if you miss yet are still close enough at minimum you will damage their engine, bust a track, blow tires, or better yet disable their main gun. If by change you hit the fast mover/soft skinned gv you WILL destroy it without question regardless of where you hit it. The same can not be said for AP rounds.
So again, I highly suggest to NOT take %100 AP ammo. Take the smallest amount of HE and use them against soft skinned gv's. For those of you worried about running out of ammo... dont be. It will be very rare that you either run completely out of ammo and/or you cant get someone to bring you gv supplies.
