Um, I work for a clothing design company (we call ourselves a manufacturer but the actual sewing is done overseas). Our specs for measurements change almost every single season. Sometimes it's because the "trend" is predicted to be either a looser fit or more form fitting but most often it's because the owner of the company wears a size 10. Period. If she's been on a cruise for a month she still wears a size 10 when she comes back
I have a suspicion that most lines are designed that way - someone in the design department likes the way a certain fit model looks so what ever measurements that model has becomes the size 10 (which, by the way, is NOT a plus size - at least for the sportswear industry).
As for the frustration with different sizes in the same store - Companies like Gap, Coldwater Creek, Talbot's, etc. actually don't manufacture a lot (if not most) of their own stuff. They contract with companies like the one I work for so they can save on design costs. Their buyers may decide they like the styles from 5 or 6 different manufactureres. Some of the companies may be European and using a different standard than a US company or some may have an owner that's been on a cruise

Paying someone in their company to sit down and block and grade a size chart then insist that their contractors tailor their designs to fit that chart would result in increased costs. There is a genius somewhere (probably a man) who said it is more cost effective to risk the loss of a sale than to spend the money to insure continuity in sizing.
I'm sure it doesn't make anyone feel any better so by all means continue to vent

but I hope that makes the whole thing less of a mystery.