Bid and ask spreads can be quite wide- up to a few % of the total trade size. So even tho you may be mid way within the range or even close to the bid or ask because certain exchanges never see your order - it gets filled at the bottom of the range- or worse never gets filled at all. The practice would be fine if there was transparency on it but most customers don't know what's happening and who is getting rebated and how much and that their orders are not going to all possible exchanges.