I suspect my reputation has decreased before without any visible notice, but now I'm sure of it. I had a 1296 reputation score 3 days ago. In the middle of the weekend it dropped to 1292 (-4). At first I kept thinking that I was wrong, I've never had 1296, because the recent achievements icon I see on the top of the site did not show any decrease in my reputation in the last 7 days. But today I accessed Travel SE from a different computer and before I refreshed the page there was my 1296 reputation score. Naturally after the refresh it went to 1292.

In short, my question is: why can my reputation decrease and it not be show in the recent achievements?