I trade options with Interactive Brokers and been seeing some strange execution behavior. Can someone with more knowledge and experience explain if this is legitimate? I had an order to buy a FOTM SPX option for $0.05. Mine was the highest bid and the only order. Yet, after a couple of hours, there was a trade for $0.03 and my order was not executed. I called IB about this and was told that the trade for $0.03 was part of a spread order and that is why someone else' trade got executed while mine did not (even though I had a higher bid for it). Is this legitimate? Doesn't the NBBO require my trade to be executed ahead of the other order since mine had a higher bid even for spread trades? Any input appreciated. Thanks.