comsec question

Hi guys,
just a question, did it ever happen to you, after you've placed an order of say 1000,000 of XYZ at .50 and you are 20th in the cue, from the top order (at the same price), in that same instance all of sudden they manage to buy for you (jump the cue) only say 75 shares.
How can they jump the cue?
Do they do it, so they can get the min. brokerage from you in case you never reach the top line?
Can they do this?
Thanks in advance!