Sub pennying is often not about making less than a penny between the bid/ask spread. It is about steping ahead of a real order by a minute amount. Say I am bidding 3000 aapl at 299.10. The sub pennyer will bid 3000 hidden .01 ahead of my order, once filled they will sell it out on another exchange at a higher price often pennys higher. Alot of time they are selling their stock to the original buyer who never got filled and is forced to either go to market or sweep the offer. If the sub pennyer gets filled and the market goes against him guess who gets filled?
