I use Rogers modem SMCD3GN. One day my nephew with his new Macbook use air mirroring with my Apple TV.. On that day my data usage was way up than my average use. More than 10GB. My average is 2GB. Does Rogers use your data usage for Air Mirroring?
A am assuming your talking about AIRPLAY? Where the apple TV mirrors what is on the mac screen, etc?That i know of, no it should not use any data in that way.. its is suposed to be all LOCAL internal on the network (hence why you cant use it when outside of your home network).
Did the usage go back down after he left?Its possible something else was updating, downloading, running on his mac while he was there (torrenting, etc).