I don't have figures, so it's just a possible hypothesis, but how does average stage length compare? Given the same number of RPMs on a shorter haul means more opportunities to sell products. 5 bags going 500 miles generate more revenue than 1 bag going 2500 miles as the fee is the same either way. Of course 2 shorter connecting flights still only generate the same fees as 1 longer non-stop flight, so total origin/destination journey distance would need to be figured. Either that or just the number of passengers, which isn't proportional to total RSM due to varying lengths of haul.