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 ...
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.