Burn and poison effect

[tofucsek]

Currently I am playing mage on impossible level. From lvl 8-15 I was only using a single green dragon to win battles (Lvl 3 order magic with Heal and Summon Phoenix- to burn them- did the job).
My question is: burn and poison effects seem to kill about 10% of the effected units per round (depends on their resistance) however the very same debuff did only 2.5-5% damage on the same enemy troop later in the game (Elon, Umkas). Has anyone experienced the same difference? Thanks for the answer.