said by mlerner:
It would depend how you setup the Apple TV. If you downloaded the movie on your computer through iTunes and setup the sharing, it will go through your LAN. If you are playing the movie on your Apple TV downloaded on your iTunes account you are probably streaming or downloading it from Apple's servers.
I have not played around fully with an Apple TV so I can't say why it did in your case but best to call Apple and double check the steps to stream from your computer.
If they dont know the answer, thats not an excuse for them to make it up.
The OP should be fine. To be sure, the OP should check their usage meter online and look for a sudden spike in usage.