Well, don't forget that the Cayman IS marketed as a notch ABOVE the Boxster series.
Also interesting is the fact that when you look at the Boxster line, more people choose the base version vs. the S model. This is in line with the idea that people buying in this price range prefer to save a buck or two if possible. However, jump to the 911 Cab, and there are about twice as many people who choose the S version over the base cab version. This then, may coincide with the idea that buyers in this price range don't care about saving the extra few bucks, and go straight for the higher status S model.