Rating changes were re-calculated, and probably some people were caught cheating. As people have pointed out before, 1000/18000 is a better percentage than 1000/17000.

same mine decreased by 15 but last time this had happened it was corrected after some time so let's just hope for that.

Mine too (from 1673 to 1666). My rank was 271 earlier and 264 now. So apparently 7 people above me were caught cheating (this should improve my performance), but usually the amount of cheaters is much larger in the lower rankings (and this should decrease my performance, because my rank is now out of lesser people).

I think due to many cheaters in the lower rankings, it leads to an overall worse performance for me. But okay, its very good that codeforces attempts to remove cheaters, even if it is at the cost of some rating points. Hopefully these cheaters will just develop a conscience and stop jeopardizing contests and bothering codeforces admins, as well as the contestants.