It's because convoy's zig-zag, and any competent zig-zag pattern should be very difficult to predict exactly even after plotting their course for hours. They could get their base course and be pretty certain where they were going to pass, but to set up to ensure a forward torpedo shot meant that you were risking getting "caught in left field" if they zigged away from you on your firing leg.
Subs got into position so that regardless of the final zig they would have a minimum range shot from either from the bow or stern. That is, that if that zig was to port or starboard it wouldn't make much of a difference.
EDIT: I should note that I've never seen anything other than a very basic zig zag pattern in SHIV, so it's somewhat moot... unless it gets sophisticated in the latter part of the war, which I have yet to play.
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