This is a result of the common Maker/Taker pricing model. [1] Those who "make" liquidity by publicly quoting ask/bids are rebated fractions of a cent when thei

This is a result of the common Maker/Taker pricing model. [1] Those who "make" liquidity by publicly quoting ask/bids are rebated fractions of a cent when their orders are filled, and those who "take" liquidity by exercising the Maker's position are charged. This is separate to the spreads. The book Flash Boys has a very good explanation of the model. [1] https://www.investopedia.com/articles/active-trading/042414/what-makertaker-fees-mean-you.asp https://www.investopedia.com/articles/active-trading/042414/...