Don't quote me on this as I have NO CLUE what Relic's programming is like, but it might be that the health bar wrapped around back to full health because the T34 did more damage than the Panzer IV had health left, but it was a critical hit so the PzIV didn't die.
So, basically;
5 - 6 = 100.
You got lucky because he got a critical hit. You can look at the icons on your PzIV and see there's 2 engine damaged icons instead of 1. You can't kill vehicles with a critical hit.