I thought they matched to the highest price which could be executed downwards. Yours was probably the best available price being offered. I'm confused about why you thought it wouldn't execute at the price you offered.
That indicates you don't know how exchanges work. If the bid/ask on IBM is 100/101 when you put in order with a limit at $120 then you get matched at $101, not $120. That is also how Bitstamp works at least 95% of the time.
There could have always been someone who entered an order opposite you at just the same time, it never had a chance to appear in the order book because you bought it so fast.