No doubt they've a method when to come/go, and how much work to do, to maximise profits..
You can't do this with PPLNS. That is the whole reason PPLNS was invented, to prevent pool hopping.
Pool hopping was a problem in the olden days when pools used the proportional reward method. You can read about it here:
https://bitcoin.stackexchange.com/questions/5072/what-is-pool-hopping