I've read somewhere that the crush depth is affected by the strain the boat has taken over it's life, resulting a cumulative "damage" or hull weakening. Meaning that the fist time it might dive up to 280 meters, but the next time a little less and so on. Also several dives in 200 m should also have the same result.
Finding out the crush depth should not be an exact science, because although the manufacturing processes were the same (in the same factory), individual lots of steel for the hull might not be up to standard specs; this also applies for other materials and the welder's experience and techniques.
Also, depth charges indeed posed a threat to the hull integrity and gradually as the uboats aged their crush depth should deteriorate too.
I wouldn't be suprised to learn that some brand new uboats collapsed at 200 meters or managed to stay intact at 300 m.
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