Interesting, could you please elaborate what you mean with that? Which brokers? And how do they get a better fill than anyone submitting orders electronically?
There are brokers on the NYSE that work orders similar to a VWAP. There is no guarantee, but they can often do better than the VWAP because they can match on all prices. That means when they are matching the bid or offer, they can get a small match on each trade. You don't have to wait for it to be your bid or offer to participate.