Removing luck and all those mysterious variances that everyone talks about, which worker received a better return?
You're talking about a very short timeframe. In that case luck is the only thing that matters and you can't remove it.
Small pool lucky: you get a big chunk of coins paid out at the right time when the price is high. You make more than at the big pool.
Small pool unlucky: you get paid big chunks before and after the price spike. You make less than at the big pool.
There's nothing new here. At small pools luck has a big effect on the short term earnings.