Because IB would have routed it to the exchange where the bid was located first. I've had similar experiences because there was a quote that was erroneous and persistent (never went away unless you tried to hit it, and when you lifted your order the quote would come back).
Not much HFT in the after hours because of the limited liquidity. Plus the dealer wouldn't see the order until it arrives. No matter how fast their feed the initial order ought to fill the posted size.
You get paid jack shit compared to the market rates a finance attorney could get and armchair quarterbacks constantly call you inept oafs for both prosecuting and not prosecuting cases, and you wonder why the best and brightest don't go work for the SEC?