We recently completed a trip with mixed DL and AF flight numbers. All of us were on the same reservation, same itineraries, same classes of service, same ticket cost, and identical cost breakdown. However, the MQDs on the DL flight number segments were inconsistent by a few dollars.

MSN - DTW, DL flight number: Me (DM) MQD=$86, Wife (GM) MQD=$81, Son (FO) MQD=$81
DTW - ... - DTW, AF flight numbers: All posted consistently as 40% of mileage
DTW - MSN, DL flight number: Me MQD=$82, Wife MQD=$79, Son MQD=$79

I understand that the difference between MSN-DTW and DTW-MSN is because the outbound was slightly more expensive than the inbound for whatever reasons, and that's fine. However, I'm baffled by the variance between travelers on the exact same segments. Any ideas?