I killed someone after the game ended

This is the second time I have seen it happen, and the first time I did it myself. I managed to kill using W after the Nexus was destroyed. In the Match History you cannot tell if I got it right before or right after (unless the order the little boxes appear mean something in which case I didn't know that.) When I go to my in-game match history however it shows a much better view even though it doesn't list the Nexus being destroyed.

So my question is, is this a bug or just a very very rare occurrence that seems to be more of an easter egg than anything?