Hmmm, a somewhat late response, from a friend of mine in The Other Portland.
He picked up SP a few days ago, and wonders if Mongoose has been allowing for the thickness of the covers, or if they have been going off of thickness of the bound pages instead. Chris is a professional layout and graphic designer for magazines, and might actually know what he is talking about*.
The Auld Grump
*He also is the fastest cardstock terrain builder I have ever met, and a few St. Patrick's Days ago he managed to turn this-
Into a hat celebrating St. Brendan.... (And won a prize for it at a local pub....)