There's an entire support section on BTC Guild related to the PPLNS, and a section devoted to the Charge-Up/Wind-Down periods that all PPLNS systems have.
When you first start any PPLNS pool, your rewards do not increase rapidly since you do not have shares in the entire 'N-shares' window. BUT, if you stopped mining at any point, you continue getting paid until your recent submissions have left the 'N-shares' window. There is no loss, it's simply due to how PPLNS works. It is a time-delayed payment method, shares get paid multiple times until they have matured.
But the short answer is "variance". If the pool has a run of bad luck then your earnings will be less than anticipated. It's completely normal and expected for all pools to have periods of bad luck.
With only 7 hours of mining done, variance isn't the answer in this case. Variance is why your rewards might be much higher or much lower than expected. This user hasn't even had a share reach maturity yet, so there isn't even an opportunity to compare expectation vs actual reward.
