Just a guess but the R recirc fan is standard on all series of 737 whereas the L is a kind of "add on" for the longer -4/8/900's. So maybe the spec is not as high because it is not as important. Or maybe the spec was written at a different time and nobody has noticed that they are different.
The only operational difference that I can think of is that the R recirc fan has a link to the overboard exhaust valve. If you switch the R fan off (not L on the -8/900) the cabin RoC will increase as the overboard exhaust valve opens.