This question already has an answer here:
Here is what my Achievements dialogue looked like earlier today.
These three items were highlighted in blue: a +10 (one upvote on an answer), a +15 (three upvotes on a question), and a -1 (a downvote I cast). The sum of these should be +24 (or possibly +19 or +14 if I had "seen" one or two of the question upvotes already). How could these three sum to +1?
The browser window for this screenshot is >1200 pixels wide, so I don't think it's a compression issue where things are getting cut off, and it's an up-to-date latest release (44.0.2) of Firefox.
This seems like one of those things that will either be obvious to someone other than me right now, OR a real bug.