Why are liquidity suppliers willing to provide "price improvements" when executing orders?

External question — Personal Finance Stack Exchange Author: Flux Original post: https://money.stackexchange.com/questions/133103 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. US stock brokers often boast of "price improvement" when speaking of their order execution quality. e.g. "95% of executed market orders received prices better than the National Best Bid and Offer (NBBO)". How does this price improvement even exist? Aren't stock brokers and their liquidity suppliers profit-maximizing firms? Why don't the liquidity suppliers take away the price improvement to make more profits?