I agree that it all depends on your butt and some will be happy with a particular product and others may not be satisfied; but, if your sit bones measure 155 on the Specialized meter, you're a 155. A cm is a cm just like an inch is an inch. We're not talking variances in shoe lasts which are not standardized, we're talking about an internationally recognized measure of length.
I think you're confused about what the Specialized Ass-o-meter does.
It does
not tell you the distance between your sit bones, at least not with any accuracy. Rather, it is a highly inaccurate piece of foam
that suggests which width Specialized saddle you should purchase. Contrary to what you seem to think, the measurement of saddle width is
not standardized. For some manufacturers, it's the width of the piece of leather that covers the saddle (i.e. it includes the sides). For others it's the width at the rear of the saddle. And for still others it's the width of the saddle at some ill-defined seating position. Because of this, there is
no guarantee that the saddle width suggested by the Ass-o-meter is going to apply to any saddle brand other than Specialized. Trust me: I've been measured on the Ass-o-meter and the saddle I now ride, and find comfortable, has nothing in common with the width recommended by Specialized.
Also, keep in mind that the Ass-o-meter's recommendation is merely that:
a recommendation. While the Ass-o-meter suggested that I need a 143mm-wide saddle, the only Specialized BG Fit saddle that I found remotely comfortable was the
130mm-wide Phenom that came stock on my Specialized mountain bike!