I ran a bit of an experiment this morning.
I opened limit buy, the limit buy was listing as active.
I then placed an instant order for the same amount, on the same stock - much to my surprise the order went through at the price I had the limit buy at!
I then repeated this once more, same thing happened, instant order goes through immediately, at the price I had the limit buy at. Then I cancelled the limit buy, and put another instant order through for the same amount, also goes through at the same price.
Do they only try to fulfil periodically??