I suspect that early in the war the RN's depth charges in-game reflect the historical record to some degree, in that one doesn't have to go as deep to stay under them. 100m seems to have been plenty deep enough on my first few patrols and even as late as summer 1940 I've avoided damage just by dropping to 70m and going silent.
Apparently British pre-war intelligence had underestimated how deep German u-boats could safely go, and their ASW weapons reflected this. Consequently the depth charges they had simply weren't designed to detonate as deep as they needed to in order to be effective against a u-boat running at its own maximum safe depth (or anywhere close to it).
I don't think the Brits fully realized how off their intelligence was until they actually captured a u-boat and found out what it was capable of.
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