There is a certain probability factor to mine and sometimes earnings will be lower than the average. Under the PPS mode, you can get 100% of the expected earnings with the subsidies provided by antpool. That is, when your miner is connected to antpool, your earnings will be directly based on the contribution regardless of luck figure. PPS charges 2.5% fee.
Under the PPLNS mode, earnings are completely correlated with the antpool revenue. That is, when your miner is connected to antpool, your earnings will be based on the average contribution over a period of time even if the fluctuation of antpool luck figure. PPLNS doesn’t charge any fee and users take risk by themselves.