I just don't understand why HD Theatre looks so good in 1080i and other 1080i looks subpar?

I know just enough to be dangerous. When I set my HR20 to Native I believe it displays the actual transmission signal received. Discovery HD Theatre shows 1080i, ESPN shows 720p, NGC shows 720p, and all the upconverted channels show 1080i. The ESPN & NGC 720p shows look pretty good too. Why...