Well. I watched a show on Equator. It basically had two parts, the first was underwater, then the second was in Guyana or something. Sorry not to be more accurate here... but:
The first part of the show (underwater) was absolutely breathtaking! No skeeters, breakups, or color banding. Lots of dark areas, then sudden flashes, etc. Nothing caused the picture to falter in quality. It was awesome! Then the second half of the show started. I noticed it seemed almost out-of-focus. I knew immediately it was from a different source because all of the captions were in English and Japanese. The quality was still OK, but not even close to the first part of the show.
So basically, I am saying the quality will change drastically based on the quality of the source material. This is a great show and I really wish the second half was as crystal clear as the first. They showed some incredible footage, including the world's largest waterfall - over 1 KM in length!
So YES, I have noticed better quality, then noticed not-so-good quality on the same channel...even during the same show!
Andrew