Your beer most likely was in a mini-best of show, where the 2 or more flights were compared head to head. Your beer scored well with the judges in its flight, and the other judges may have been grading tougher in their flight. The mini-bos is the way that the lead judges from each flight can taste the best beers,and decide which ones advance.
It is tough to have a beer score so well and not go on, but it happens.