Could be a ton of different factors.  One thing to consider, is theres about a 12hr delay in payouts.  What we show for earnings today, will realistically not be paid out until 24hrs later (~8-12hrs for coins to confirm, time for transfer to exchanges, exchanging, time to transfer back, and ~12hrs for payouts).

It is also extremely possible to have happen based on variance.  I don't know the speed of your miner in particular, but if you randomly got unlucky (share wise) on certain hours where the pool got lucky (block wise), you'd be getting paid less during that time, and if you happened to get lucky (share wise) when the pool was unlucky (block wise) you'd be getting paid less then.  PPLNS tries to mitigate some of this, but everyone will still have periods of (un)luckiness.