My decaps on the achievement tab of my profile says that on my 25 decapes was 19 but i checked it a few days ago and it was 12 and i thinki havd even more than that

First of all general reports is wrong board for this, for technical help about toribash you should head to Support board. I moved the thread for you.

Anyway, two things to consider about achievements:
1. The description goes "You decapitated 25 ugly heads in one week - cool."
2. Aside from that the achievement system has lots of wacky moments when they don't record things correctly.

So your problem might be a mixup of the two. Anyway achievements are still a mess, to be honest.