Random reward amount?

Why the amount of rewards for a found block is different? For example dogecoin's pools. Some times the amount of reward is way lower than 500,000 coins.
28418 block has 326,184.03 dogecoins...
19587 has 774,036.00 which is actually way higher than 500,000.
IS this only for some reward systems like PPLNS?
Especially in Dogecoin I saw something very strange - mining in low hashrate pool (5-10 megahash/s) seems to be WAY MORE PROFITABLE than mining in big hashrate pools (1000-4000 megahash/s).
Sure I wait a lot to find a block but the % I gave to the found block is way bigger than from the bigger pool and the bigger pool is also slowly validating my rewards - it seems the big pools also take a lot of time to found a block. Miningpool.co has around 40-60 megahash/s and check this Pool Block Time 11 Hours 52 Minutes 50.2 Seconds.
But the low hashrate pool (currently 8.8 megahash) gets a block in around 24 hours!