I always associated it with the early 2000's Australian attack:
Lee as RF, Gillespie as RMF and McGrath as RFM. And Gillespie was clearly quicker than McGrath by that stage.
Although cricinfo lists Gillespie as RF. I'm confused.
Sometimes it appears completely random. Chris Woakes is still described as RM despite pushing 80mph quite regularly in recent times. You can't read to much into the classification, really. Judge by the speeds on your TV.