I'm assuming the orders in front of you were on the same price level.
Check to see if the trade was a cross trade (usually "XT" on the trade type, depending on broker.)
Without knowing all the details, what I suspect happened is that your broker had matching order come in from another client at that price level and executed it internally as a cross trade.
This happens quite often.
Your broker should have a publicly visible "Execution Policy" (or similar) outlining the strategies they use to fill the order....some brokers allow you to change it.
Generally, the policy is written in such a way as to reduce costs to the broker...so it looks something like.. 1) cross trade if possible - this is an "in house" thing, so they save some market costs. 2) Chi-X if possible - cheaper than ASX execution 3) ASX
If you get a chance, speak to someone at the brokers, and make them earn their money !!
