PPLNS ("pay per last N shares/shifts"), pays the last N amount of work, for some number N. It may feel unfair if you did some work and it was no longer among the "last N shifts" when the block gets found so it didn't get paid. Then a big miner arrives, does a lot of work, after which several lucky blocks are found, so their work gets paid multiple times. It may feel like they are using a loophole. In fact it is completely random and could easily have been the other way around.
Guys, I'm obviously not explaining myself as well as I should so I'll stop there before I annoy.
I understand luck/prediction etc.. And that in my example it could've happened the other way around. The example was just to point out PPLNS does have a flaw - I wondered were there other quirks?
I understand as well if there's no reason he comes and goes.. He may be bored, his other pool may be down, etc.
I suppose in it's most simple form my question was just:
"Is there any little quirk of PPLNS he could be taking advantage of, reason he comes/goes?"
In it's most simple form the answer would've been:
"Nah.. Probably has the pool as a failover or he's trying out some different ones, etc."
His coming/going doesn't bother me at all, I'd rather have lower payments more regularly. There's no ill feeling anywhere, just idle curiousity.