The gist of it is that RH was cheaper than other brokerages at the time for small trades, and more expensive for big trades (because it dropped the commission (independent of trade size), but offered a somewhat worse price improvement vis-a-vis the national best bid offer, pocketing the difference (proportional to trade size).

It's not especially heinous, but they were not very transparent about it (to be fair, it probably would have taken a bit to explain that they took a greater cut of the price improvement vs the legal benchmark).