It all comes down to how many blocks were found during your "Shifts". As shifts fall off without finding a block (which happens to many shifts during a nasty 97% CDF block like the last one) those shifts do not receive pay since no blocks were discovered. With this pool, only the last 10 shifts are paid when a block is found. For example, in December, during that block the total TH/s was 2.7 Ph, while in the most recent the total TH/s was almost 8 Ph. This means that the % of shares per shift was significantly smaller unless you also increased your hash rate during the jump. As of now the current hash rate of the pool is hovering around 4.5 Ph/s. We lost a lot of hash rate after that last block so shifts will take longer to complete, but at the same time, your percentage of submitted shares will rise and the amount you are paid will theoretically be higher than it was when there were many more miners and a higher pool hash rate.

The only way you can expect the same pay on two blocks would be for the pool hash rate to stay the same, and for the block reward to be around the same size. I'm sure in December there were a lot more Transaction fees than there are currently which is a small factor in differing pays.