Any idea why I've got letterbox watching football with 16:9?

I've got a new Epson 8350 connected through TiVo/HDMI and I still have a letterbox watching football today on Fox. Its sending 720p from the TiVo and the aspect mode is set to Auto (it won't allow a change since its HDMI).

I thought it as broadcast in 16:9?

I'm going through a Monoprice 1x2 splitter if it makes a difference. My TV on the other side of the splitter is 720p.

Yep. When I switch to SD it automatically switches to full screen. It switches back to letterbox when I go back to HD.

It should fill the entire 16:9 screen, right?

Figured it out. Stupid TiVo video aspect setting. It was set to Panel 16:9 instead of Full. When I set it to Full it worked. I assumed Panel would output correctly!