We have the various escorts set-up differently in equipment and weapons, and try to spread them about the place, but you will find convoys, troopship groupings and task forces sometimes with very deadly DD / DE combinations. Even some of the Frigate / Subchaser combos work just a pinch too well together if you find them with some of the smaller merchant groups. We did attempt to limit those combinations though, so that a player has a chance to make it through the war - if we don't CTD the game on you...
But just as the "skill level" will fluctuate up and then down on the escorts, and as you see more of them, and fewer ships as time progresses, so also fewer larger escorts, and more smaller ones will be found. Then you have to go along the coast of China or Korea or Japan to find more / smaller targets, including sampans that if you're too close, will take you out with them as they explode...