My AppleTV uses the same amount of data when I use Airplay as when I stream the same content.

When I watch a movie or TV show that I have downloaded to my iPad on my AppleTV, it uses the same amount of data as if I had streamed it instead. I have satellite internet, so it is limited. I have tried switching to my phones hotspot and watching it, and have the same results. If I am watching a dowloaded movie over Airplay, why is it using any data at all?