chaadster

Well, at some seat width, sitz width is irrelevant. Armchair comfort isn't dependent on width, for example, nor is a park bench, beach chair, stool, etc.

The issue is of how you sit on the seat and where the pressure of your weight is distributed. A narrow bike saddle that fits between your legs will bear weight very differently depending on riding posture and position, and is obviously different from the way you sit on a flat chair.

I think for most sport riders, there is no relationship between the results of measuring width between ischial tuberosities as recorded on flat surface, and saddle width across the middle of the rear portion (or flare, maybe they call it).

I don't know what the Specialized measuring system is, though, so unless it's somrthing like the "dents in the cushion" type system, I can't comment on it specifically.