Well, the burn/poison effect is based on a formula that you can find in the KBAP info thread. There is some randomness in it, so you probably just got a bad "dice roll", or the troop was bigger or smaller.
To save you some time, here's the formula:
Damage = Burn/Poison Factor * random number (0.05-0.1) * Current total troop health * Medal Bonus
Additional info on this subhect - here, Part X:
http://forum.1cpublishing.eu/showthread.php?t=10991