The most likely possibility is that not all channels you are watching are true HD. Most channels (PBS is the exception) don't broadcast HD 24/7 but rather for special events and primetime programming.
For me,anything with black bars at the sides of the screen is just an upconverted SD signal, but anything that is broadcast in 16x9 is HD.
Also, I've noticed that some channels don't look that great for some reason... ABC seems to not look as good as any of the other HD channels, but still WAY better than SD.
Also HD has a really wide bandwidth, and I got rid of many of the problems I was having at first by tossing the cables that came in the box
