United does something similar on their A319s / A320s. Economy starts in row 6, even though the A319 has two rows in F and the A320 three rows in F!

I have always assumed it allows them to sitch equipment according to capacity without having to issue new seat assignments to everyone.

However, it does not seem that AA does anything similar ... eventhough Y on both the MD80 and 737 starts at row 7, you would still have to re-seat everyone in a B/C seat!