I don't understand the people who are higher SLs in 8 ball than 9 ball. But I know a few. The oddest are the established 8 ball SL7s who are SL 6s in 9 ball. Although from what I can tell, it's often guys who have been playing APA 8 ball for 20 years and only recently picked up 9 ball and perhaps still don't understand 9 ball pattern play.
We have a few of those in our league, even one on my team. Most of them are newer players who's skill level just hasn't really hasn't caught figured them out. All of them are borderline between the next skill levels. We do have some "lifers" that are a 4 in 9 ball and a 3 in 8 ball. I typically find these are the ones who just hit the ball way too hard and get lucky in 9 ball, or they are coached to play safe instead of take high risk shots (like most lower skill levels will do). So in the end, the opponent gives up ball in hand and they get straight in shots to artificially get balls down and decrease the needed innings to win. In 8 ball hitting them hard is really a detrimate to their game because it moves their balls away from the pockets and its a lot harder to get ball in hand because the opponent has more options to hit on the table.