Return of Reckoning

Help me out here - ive noticed that a few abilities on my engi are dots over 21 seconds ie 7 ticks at 3 secs/tick. Magus mirror ability is over 20 secs. Does magus still get 7 ticks or 6 making each tick more damage assuming the same damage as engi?

To begin with, Magus version does 22 points of damage more. And yes, I'm assuming the duration is identical, the difference probably comes from inconsistence in classifying the last tick as next second or not