Maybe they have a minimum depth that they are at, so that they don't blow the ass end off the destroyer that dropped them?
Also, I think ASDIC had some trouble tracking targets running shallow, at least early war. Maybe that has something to do with it. And it lost contact for a bit as it passed over the target before dropping its pattern, so that might have something to do with it as well, if this is represented in game. If you are maneuvering as the enemy closes in, this lost contact could throw their solution off.
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