It is likely your local station not fully used to HD yet. Last January, I noticed some quirks with the local CBS station deciding for a time to expand all 4:3 content to 16:9, but eventually they backed off doing that. I'd also notice odd issues at times when they would switch to commercials, as they used to switch to a lower resolution for them, and not always switch back quickly when the commercials were back.

The local FOX station seems to be the best with HD, though usually commercial breaks sound odd when they try to upsample the sometimes mono sound to 5.1 to match what was on at that time. They stopped changing the sound format during a show as they got complaints that some people would miss the first few seconds of the show returning from break as their recievers switched formats.

ABC and PBS here are still only digital, and not HD. NBC and UPN still lack even basic digital transmitters.