That is really an IHub question. It has been asked here over the years. If that old explanation doesn't answer your question fully, I suggest you ask Ihub.
Just had a trade surprise me, when I cancelled the sell order it immediately showed up on my time n sales and delayed Ihub > ? < section showed a trade, never sold a share that day. WoW!!!
Sell 1250250 DNRG Cancelled 12/12/18 | 03:35 PM ET