A lot of people can not just keep lapping the lifts all day either, not the same runs but just straight riding, they tire out by 1100 when I do ride with them and I'm still good for 15+ more runs....
Argo has it right. People get tired, have to take a lunch break, change clothes, etc. That's when the lift lines get shorter and you hav less people on the slope. Last time I went with a couple of friends they had to break for lunch and wanted to know if I was hungry. Nope....that's why I have pockets for some food and a drink stash on the hill.
Expect the unexpected and then deal with it.