The fact remains that GPA measures CGC sales, which anyone with a brain must admit, are a combination of many different factors - label chasers, registry hounds, speculators, collectors, etc.
When you're talking raw, a lot of those potential buyers get tossed out, and therefore make across-the-board data comparison impossible.
The only time GPA data should be used to promote and sell raw books, is in cases like Roy's, where he makes a "money back guarantee" if a book fails to grade X.X when submitted to CGC.