As the achievement and the Heroic Deed's requirement slightly differs you such a difference in the counting can happen. If you deal with traps in a given mission but the mission eventually gets lost or you abandon it etc. the heroic deed won't count the dealing but the achievement does count it. Please try it out with a few more traps you should be able to unlock the DoT skill tree.

We are looking into this right now, thanks for reporting the problem!