http://www.psacard.com/ shows that they've graded/certified 24 million items since '98. So a million cards a year just through PSA wouldn't be surprising.
There might be minor edge/corner wear on a 9 compared to a 10. Base cards from a modern factory set should all be 9s or 10s unless they have some kind of defect. I saw a modern card with a smeared numbering stamp that was awarded a 3.5 recently. Not sure why someone sent it in for grading in the first place.
Modern cards can also have surface issues that might knock them down, or they may have gotten dinged in the packing process. Cards with windows to see the jersey/bat sometimes have creasing around the window area, that knocks them down to about 6s.