I get better gas mileage by foot rather than cruise. Seems I can get the the transmission to downshift by foot at the same speed the cruise wants to keep the RPMs up.
I also do too much around town driving causing the need for regeneration. Notice a significant drop in mpg just before a regen occurs. So it seems to me that if you are starting off a trip with an exhaust filter that is dirtier than a previous trip, this could account for variations in MPG.