Hey Despy,
I'm pretty sure the month with "R" thing is for shellfish, predominantly oysters, not for crabs. Oysters convert a large percentage of their body mass to sperm during the summer months hence are very liquidy rather than being firm and plump as in the winter months with "R"s in.
Crabs grow my molting their shells. A soft shell grows under the large one which then then splits and is discarded leaving the soft shell to calcify as a larger shell. Soft shell crabs tend to have little meat in them. Young crabs molt their shells fairly often with the process slowing down as they get older. I'm fairly sure they molt year round so I think you may just have been unlucky enough to catch a number of soft shell crabs during the summer months.
Anyone disagree???