Possible ground combat bug??

[Suicide Junkie]

I was under the impression that damage from all units (that won the roll to hit) was added up and fired as one burst. Any leftover damage was lost.

I had one ground combat going, where 10 of my tanks simply could not kill the one enemy militia. (I knocked out all the units they built each turn)
After 6 months or so of fighting without losses and without a win, 10 more tanks arrived.
After two rounds of ground combat, they finished off the one militia.