said by ChuckcZar :Because you have your Bell receiver set to output in 1080i.
My Hauppauge WinTV HDPVR Video Recorder reads the signal as 1080i. I don't know how you get 720P.
The receiver converts the signal from its native 720p to 1080i when you have this selected. You can change this by pressing MENU - 6 - 7 on the Bell remote.
Nevertheless, the native Bell feed is 720p.