Perhaps the round is almost completely 'defeated' by the armour plate and thus penetrates only a little, perhaps the HE burster(if the round has one) explodes too early, perhaps it enters into a part of the tank where it doesn't damage the crew, perhaps they get lucky? Who knows, but it is a hell of a lot better than, say, the 'Close Combat model' where every penetration ever achieved results practically always in a catastrophic explosion.
It should be more prevalent with small caliber ammo and those without HE charges, especially if they can only manage extremely meager partial penetration - I seem to recall a pre-war Soviet study indicating that the relatively low calibered guns of the time were estimated to require several shots to hit and several hits to penetrate and several penetrations to destroy.
[ January 03, 2003, 03:07 PM: Message edited by: Mikser ]