No. there is always 15 reputation is awarded for acceptance of one of your answer regardless the question is open or closed.
-2 should be for a downvote on one of your post or -1×2 for your downvoting to couple of post etc, though there are also other possibilities of voting on your posts that may result in +13.
Just click on achievement button to recheck it. More precisely check Reputation tab from your profile activity page to understand any chances in reputation.
Anyway, the current page of your reputation history doesn't show any downvote or +13 recently. So, it may be undo of any voting operation by a user.
In addition to this, you may also supposed to check the reputation history on your other SE communities/sites as the achievement popup may have combination of your reputation changes for multiple communities.
Tip: You can audit your reputation by clicking here!
This MSE post may help for users who had 1 reputation.