I found a pretty good mileage based fare through EF and I’m trying to figure out what's happening when I try some different routings in ITA. I understand that there’s a MPM associated with the fare that I can see under the routing details, however what I don’t get is why, with certain routings I'm trying, I can still go above that maximum and sometimes the result is a fare increase and sometimes not. What confuses me is that just the fare itself increases, the fare basis stays the same every time.

Also, let's say on the outbound I go 2000 miles below the MPM, can that 2000 be used on the return without a fare increase?

TIA