Can it be that masternodes.online using wrong data about circulating supply and so has wrong calculation
masternodes.online gets realtime data from blockchain, so it is very accurate.
@ice2090, read this:-

This is my MN. Please pay attention to Amount and Date. As you can see that I did not get a lot of rewards in the latest MN payout. But please check the earliest MN payout with 270+, anyway you will get an average about the same as you see the stats from masternodes.online. The MN rewards are vary in time.
beybietron has explained it clearly:-
The dev's premine is not the only one that's staking. There are other people who stake as well, so you will get a series of high (from premine) and low (from other people) rewards. At the end of the day, the average would be roughly as what the estimated ROI stated.