ComicLink works the same way that eBay does in their bidding.
What's important is what the bid was before the winner made his last bid.
If $5,311 was above the minimum increment extra that he had to bid, he is allowed to bid and that will be his winning bid because it's above your $5000 proxy bid.
For example, if the bid prior to the last bid was $4700 and the minimum increment is $500, all he has to do is bid $5200 or above for the bid to take.
The system doesn't require them to bid a full increment above your max, as it's unknown to others at that time.
Hope this explains everything.