If I ask a question, answer that question and accept my own answer, then when I hover the mouse over my username at the top of the page the dropdown summary will display that question together with zero as "reputation gain". I guess it should not display that question at all.
Why shouldn't it be like this?
The list is showing you an event that has happened, and the resulting modification to your reputation.
By showing you it this way (with 0 change), it clearly shows you that a normally rep-giving action has happened but in this case didn't give rep. It makes it clear to you that the accept has really happened, and also that you receive no reputation adjustment from it in this case.
It's exactly the same principle as showing posts on the reputation lists that have been affected by the reputation cap. The event (a vote on your post) has still occurred, it just so happens that no reputation change resulted, but that doesn't mean the post shouldn't still be displayed in the list.