Pay with Miles tickets are just revenue fares that are partially/wholly paid with miles exchanged at 1 CPM. I doubt they would be lower than Award fares or Miles + Cash awards (which are just Award fares, but where redeemed miles are partially paid for with cash at about 1.8CPM). I suspect you are making too much out of the order of listing types of tickets which are eligible for upgrades. Delta has never really exposed in detail how "Fare Class Groupings" work with the various ticket types.