The quality of the fries is directly proportional to how good the attendant at the fries station is at following procedure and not dumping loads of pre baked fries in the keep-warm bin (don't know the English McD's phrase for it). They get worse from being under the heating lamp for too long or being left over the frying pan too long dripping. It's not rocket science but many don't want to be shouted at when the station runs out of fries so they overdo it on the supply. This is exaggerated when a rush is winding down and the production isn't scaled down quickly enough.

If I remember correctly there is a small trouble shooting section in the floor managers quality guide (small booklet with all procedures, weights, temperatures, stack height of boxes etc) which hints you at what is going wrong if you ever want to know and get your hands on one. Though that will have changed since mine is ancient.