The depth rating of subs can be a bit confusing. But if you think about it, you could hardly design a pressure vessel to be perfectly safe at, say, 300 ft, and catastrophically implode at 301ft. All subs are constructed with a crush depth (where the pressure hull is expected to fail) well beyond the desired operational depth. In WWII boats, this depth was normally about twice the operational depth. That is, crush depth pressures had a factor of safety of two. So if you wanted a sub to go to say, 300ft, (~150psi) you would design to a crush pressure of twice that, or ~300psi.

But when designing the boat, the various valves, etc would have lower safety factors because of the reduced risk if they failed. So most valves, etc could not handle the high pressure of the ultimate crush pressure of the pressure vessel. If you went beyond operational depth, various valves etc would start to leak long before the boat imploded. Dick O'kane played with this on his Tang by researching, and jury rigging private fixes to, the various leaks he found so as to increase the operational depth of his boat, which was, as pointed out, a Balao class boat (which was also widely referred to as a 'thick skinned Gato'). Also, operational commanders quickly learned that the operational depth was very conservative, and most fittings in the boats could go well beyond the operational depth. This was true for subs of all sides in the war.

There is an analogy in aircraft design. Normally an aircraft has a safety factor of 1.5 in its structre: an aircraft limited to 6g flight has a structure designed to stand 9g's (and often, when tested will exceed even that strength). This is standard to this day.