Yeah Ive always found that a bit odd that the fleet boats max depth was well under that of u-boats...
However, could the explanation simply be that the germans had a lot of prior experience with submarines, at war.
What were the diving depths of WW1 U-boats?
Edit: According to U-boat.net the UB III's (most produced of WW1) had a max depth of ~250' (75m).. so not very deep. These boats are quite similar to VII's.
http://www.uboat.net/wwi/types/index.html?type=UB+III