Western Seat Measuring

Ok. So heres the deal.
We have two 16" saddles. A Billy Cook and an Ammerman. After riding in the Ammerman I got in the Billy Cook and it was MUCH smaller. Both are marked as 16". Both have padded seats. FASCINATED by the inconsistancies I busted out my trusty tape measure and was AMAZED to discover that:
The Billy Cook measured 15" from the swells to the seat padding, 16" to the cantle.
The Ammerman measured 16" from the swells to the seat padding, 16.5" to the cantle.

Sooo... which is the standard measurment? When I buy a 16" padded seat should I expect it to be 16" to the padding or to the cantle? If my mom likes her 16" Billy Cook that is padded up the wazoo then should we get a 15.5" of a different, less padded brand? For her, the Billy Cook is more comfortable, seat size, than the Ammerman... so what size to we buy?

Thoughts?

Any comments/questions/advice?

Also, the angle and type of cantle will make the seat seem bigger/smaller. A 15 inch saddle with a short, flat cantle like a roping or pleasure saddle will seem bigger than a 15 inch saddle with a high cantle like a ranch saddle.

Hmm... interesting. Sounds to me like my Ammerman was actually a 16.5"