Grading questions (various) with photos

[Mesquite]

How do you distinguish between a PSA 9 and 10? Other than a slight difference in centering allowance for those 2 grades, it doesn't seem there's much difference, especially for newer cards from a factory set where there's no chance of staining.

I've looked at many 9's vs 10's on ebay and I'm not able to conclude much.

What percentage of a just-opened factory sets would likely be 10's versus 9's (Is it the case that a newly-opened set--if centering is good--would have no card that would grade lower than 9?)

PS, and just curious, anyone know the number of cards that are submitted for grading each year across all card grading services? Do you think it's over a million cards?