It's possible that the eventual winner produced more ships over the course of the game than you did.
This is how the tie-breaker works if there's no winner by turn 300:
1. Check which player produced the most ships
2. If that's a tie, the winner will be the player which did the most combat damage with their ships
3. If it's still a tie, a winner will be determined randomly, but that's highly unlikely to occur given the first 2 winning conditions.