Some of this is about having a safety margin.

The water-proof pressure numbers tend to be large because you get momentarily higher pressures. For example, 50m is the minimum you want for swimming, even if you are never going to swim down to a depth of 50m. Even for snorkelling you need 100m. 200m (which is your 660ft) is good for scuba diving. That's what my watch claims, and although I don't scuba I do snorkel and it's nice to have the difference as a safety margin.