I have wondered about the same thing. But I doubt temp is the difference: I ran the numbers (70 deg vs 50 deg) and only come up with a 4% difference. And some of that is negated with the increased humidity at the coast (higher humidity, lower density).Air on the coast is colder and more dense that the air in the Bay and Delta. 20mph at 3rd and I'm riding a 10M or 12M. 20mph on the coast I'm riding a 6
My guess is geography - there is more wind higher up because of the cliffs. The windsurfers are usually riding about the same size sail as they would at 3rd.
Just to stay on track, at ~200lb I mostly ride the 8m at Waddell although I often need my 11m at HMB and Gazos.