Guys, I got it- it's because Jimmy is number 22, so he has to be ranked 22. Duh!
But in all seriousness, we have to remember PFF takes into account coverage, run defense, and penalty into their grades. Jimmy is far from his early run defender self, preferring to be clean up rather than head on attacking like Webb does. He's probably got run defense grades that are pretty weak. I'd be very surprised if his coverage grade weren't top 10.
As far as corners that could be better, I guess you could argue for players like Ramsey, Peters, Sherman (maybe), Talib, Harris Jr, Jenkins, Peterson, Norman, Rhodes, and Hayward. Off the top of my head, that's all I've got.