Interesting. In this case, the number and type of ships are fixed at a small transport and two small freighters, plus three escorts. Even when I've ignored them, they made their way to my assigned patrol zone to waste more of my time.
I think I sighted them just before the save, locking the configuration forever.
EDITED TO ADD:
Just ran a series of six tests of a single save, this one later than the one described above. In this case, we were just N of Oite and the pattern observed was 1-2-1-2 where:
1 = No ships sunk at Oite and several ships detected to the E/NE on sonar only
2 = A LEO lying on the bottom at Oite and a LT a few km to the N, on radar and visual.
I'm not entirely sure which was the correct situation at time of saving, though i believe it to be #2.