In the bonus disk of CFC you have the conditions in which this happens, IIRC:
A full 4/5-hp tank may lose against a spearman fortified (25% defense bonus) on a city perched on a hilltop (50% defense bonus), with walls or being the city bigger than size 6 (to obtain the 50% defense bonus), with civil defense (50% defense bonus), a radar tower in action range (25% defense bonus), and being the city behind a river (so as to get the 25% defense bonus). So If you add all these defense bonuses together, it's no ordinary spearman you're engaging with defense of only 2. So if I'm not mistaken the guy has well over than 12 defense when you attack him under the said criteria. And tanks only have 16 attack. So it's very likely you'll lose the tank.
Use the combat calculator to see you're odds.
And then, obviously you have to take into account the RNG which at times does some peculiar things.
Check out the story The Spearman in the Stories&Tales forum.