This is strange. I just re-tuned my DVD recorder, and I think I am picking up some OTA channels with it, even though I don't even have an antenna attached, just a coax cable. All of the OTA channels are the HD versions of the channels, but any sub-channels are in SD. It's strange.
Crash course in technology:
You might be picking up what's called QAM channels...all digital cable channels are in reality sub-channels of the original 125, then mapped to big numbers. Most providers provide in-the-clear QAM with local digital (meaning you get HD channels), and possibliy some bonus channels (if you're lucky).
Also, you can have only one HD channel per OTA station currently...two makes the picture look like shit (not joking).