Looks like OP miss clicked on the tick - any reason why the +15 isn't reinstated?
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Because your answer became community wiki before accepting it again.
Exactly what happen:
2012-09-11at that time it was not a community wiki answer so you got +15 for that.
You have edited your answer more than 10 times ( which is 16 revisions ), hence your answer become Community Wiki answer automatically.
As per the rule of the Community Wiki Answer, it doesn't generate any reputation, that's why you are not awarded +15 reputation again.