We use PPLNS reward system on 2Miners. Miners work together to find a block. When it is found they split the block reward based on their hashrate. This system is used to prevent "pool hopping". Pool checks how many shares you've sent from the last N shares of the pool and makes the payouts based on that value. For example N value for Ethereum is 300 000 shares. Read more
It could happen so that your hashrate is too low for example if you got just 1 GPU. In this case even if you send shares to the pool when the block is found your percentage could be zero (you got 0 shares from the last 300 000). You will not receive any reward for this block. However, if you keep on mining your daily rewards on average should reach the calculated values.