On Super User, one of my posts got upvotes and when I edited it, someone unupvoted. So there should be -10 rep cost for this while it just cost me -5 rep. Is this a bug or there is something that I'm missing?
You reached the cap of 215, but you netted 210 for the day because of this line:
That vote was cast back on June 9th, but the voter removed the upvote after a later edit on the day in question (Oct 26th), meaning it your reputation changed by -5 as a result that day, so 215-5 = 210 net rep change.
For the updated screenshot: the reason it's a -5 is because it was from a previous day, the vote that was undone only cost you -5, since the reputation cap kicked in and got you within 5 points of the cap...meaning that the vote only really affected your rep by +5 originally. Note since the screenshot things have shifted and that vote doesn't account for anything on either day.
Congratulations! Yesterday, you hit the rep cap at 215 reputation total. Because of the rep cap, the full 10 points were only counted as 5, so the removal today only cost the 5.
Check out your reputation history for more details.
What's interesting is that you actually lost 0 rep for the post you mention, since you gained no rep from that upvote. The post where you lost -5 was What is the difference between 1080p and 1080i, and the unupvote occurred yesterday, which is why you got 210 rep instead of the full 215.
I am confused about why your other upvotes for that day aren't counted to offset that. Because you got one accept that day, 215 should be your max, and you have at least 2 additional upvotes that weren't counted...