keltos01 or similar would have to correct me, but the only reason that I can see that the rate is listed as 75%, and not closer to 90%, is because the rate you have counts the boats that had been retired and were being used in training. See
http://www.combinedfleet.com/sensuikan.htm for more. One Jyunsen B1 left at the end, or one RO boat left... Some of the classes no longer existed.