Most of us figured that the tank is mostly dead, but not all the way dead.
I haven’t looked at the source code, but perhaps health is a floating point number that is displayed as an integer. Thus, one could be at like 0.9 or 0.5 or even as low as 0.000001 health, but all of those would display as 0 health.
Either way, it’s a cool feature since it can be so DARN FRUSTRATING when your opponent doesn’t die! 😉