1) You use a mobile phone with, 20ms? latency

2) The price displayed to you is 20ms late

3) You accept order at this price

4) The actual price might be less, so a firm can buy it and sell it slightly higher.

5) You get a fill at a better price than you thought.

- There is a premium to Robin Hood vs other apps because most users are on mobile, where latency is likely to be higher.