On a stock >$1 quoted at say 4.99 X5.00 , how does a print like 4.9999 fill when smart routed? Only Nasdaq and arca was showing at the time on the inside. Who is doing the fill? I know some firms route to MM's , who may fill like that but I didn't think IB did. The fill was tagged NMS
Was it a single fill or a couple of partials? If it was partials maybe you hit an odd-lot first and then the rest was filled on a round-lot. And the average price fill for it might be 4.9999. If you look at your executions window, can you "open" your execution to see multiple fills at different prices?
That is your broker doing the filling when you see a price like that. They must beat the best ask or bid shown, so you get 0.0001 in savings. You must feel special. Basically they have another customer that is selling and they internally match buyers and seller. Your broker, not the MM or specialist, makes the spread.
Is it possible an NMS fill can include a partial odd-lot order? By the way, your attitude could use an overhaul.