This is probably quite a common situation with freight paths. This is pure speculation on my part, but I'm thinking maybe the loading terminal (Rylstone) might not be working over the weekend, the normal situation will be for the loaded train to lay over at Neville Hill on Sunday, working forward as 6D71 to ensure a delivery to the receiving terminal (Dairycoates) on a Monday morning. However, if a delivery on a Saturday is more important to the customer, the 6D03/6D04 paths will be called. Another possible reason for the paths is in case the stabling facilities at Neville Hill are unavailable for some reason. Just because a schedule exists doesn't mean a train will necessarily run!