About PSA vs CGC submitters, I can only speak to my own experience. My PSA friends are generally more familiar with grading, evaluating/buying raw cards, and carefully only sending the "best" cards. They are not sending every pack-fresh hit to make the 50-card bulk rate.
I also believe CGC has adjusted their standards the last year. CGC used to be overly harsh imo, and a majority of the 9s should be 9.5 or better. I've cracked and resubbed a bunch of 9s with 70% getting upgraded.
So my point is, I think in a year or two these POP reports may become more aligned.