Basically, rows are numbered so that, if you have the same seat number, you get the same type of seat regardless of fleet type on narrowbodies.
Since 757s in domestic config (all -300s now) have 6 rows of F, the first E+ seat starts at row 7. If they do a last-minute equipment swap, you keep your seat number and you stay in E+ (they don't have to change everybody's seat). Exit rows are always 20/21 for the same reason, then after that is regular Y seating.
That said...not sure why they skip row #9. Anyone have an explanation for that?