Damage (I strongly believe) is tracked by what hits the tank. That damage is not tracked by whether it is actually hurting the tank.
So a p51 can put 1500 rounds on a tank. Not a single one of these rounds will have penetrated, but those 1500 rounds are counted towards total damage done on the tank.
Then you come along and put 3 88's in it. While you penetrated and actually did vehicular damage, the total damage you did compared to the 1500 rounds was small.
The best way to explain it is to quantify it. Let's say that 1 .303 round is 1 point. 1 .50 is 3 points. Let's say that 1 88 does 200 damage.
If you spray a Tiger with 1500 .50's, you've put 4500 points worth of damage on the tiger, but you haven't done damage or crippled the tiger. You put 3 88's on target, you've done 600 points of damage. You've penetrated and actually crippled the vehicle.
But you have "Done" 3900 points less of damage to the Tiger, so the p51 gets the kill.
Or take a bomb, for example. 1 1000 lb bomb does 2000 points of damage. You drop two nearby and you've done 4000 points of damage - range factor. So roughly 3500 points. While you haven't hurt, stopped, tracked or taken his turret, you'll have done more damage then another tiger can do.
Now, I don't know the actual number and the actual damage done by the seperate weapons, I was just making an example.
However, I do know that kills for Vehicles need to somehow be tracked by crippling or penetrating a tank.