My answer to this question was not accepted but was upvoted once. Yet when I check my points the answer is listed as -2. What gives? This has happened twice in one day!
Your post has received 2 upvotes and 1 downvote, for a total of 1 displayed beside the post. You receive +10 reputation for each upvote and -2 reputation for each downvote.
The achievements dropdown shows reputation changes rather than vote changes, so the -2 shown in your screenshot refers to the decrement by 2 of your reputation for the one downvote received on that post today. This action took the post's vote total from 2 to 1.
- 2018-08-11: 1 upvote, score from 0 to 1, +10 reputation (running total: +10)
- 2019-02-02: 1 upvote, score from 1 to 2, +10 reputation (running total: +20)
- 2019-09-25: 1 downvote, score from 2 to 1, -2 reputation (running total: +18)
That -2 is from just a single downvote; the two upvotes you got (giving +20 reputation) were given on earlier days and do not appear in your achievement inbox, which only lists the most recent changes.
For more information, check the timeline of your answer and your reputation history.
You can also click on the number between the two up and down arrows on any post to see the vote counts for upvotes and downvotes. It is an 1k privilege iirc.