Selection of rivals is engaged by a special program - the balancer, it chooses your opponents considering many game parameters. One of them is your highest ship in the port. Depending on your experience of playing on it (amount of battles, amount of kills) the enemies are chosen. The longer you play, the stronger enemies become. If you enter the battle, for example, on 4-tier ship but in the port you have 8-tier ship, the enemies will be chosen with your higher-tier ship.