Good example is the '78 Halloween... last Halloween AMC broadcast one master on the SD channel feed and a completely different master on the HD feed. A few of us even did comparisons to see which discs matched what they were broadcasting. On the standard definition channel, AMC was showing the 1999 Limited Edition DVD in pan and scan... you could tell because of how blue the night was as well as the amber/orange of autumn. On the HD AMC channel, they were showing the 2007 Blu-ray (first BD release) obviously in anamorphic widescreen and it was clear as day because that release has an unmistakable lack of the blue night but ultra clarity and vibrancy. The daytime scenes also match the somewhat warmer look of earlier releases. Everyone who checked it out agreed on what sources they were using. So even the same network can use two different masters to broadcast the same movie at the same time but on two different feeds... one SD and one HD.
Also, I have noticed that EPIX channel uses the Star Trek Blu-rays when they broadcast films 1-6. They match the BD exactly. But when Cinemax broadcasts films like Conan The Barbarian, Jaws, Star Trek II, etc... they use the earlier HD master from their own library which does not match any Blu-ray. The color timing is the dead giveaway. Totally different. HDNet Movies does the same thing because they've had those HD masters for years. So bottom line, some networks or premium channels have HD masters that were struck long before they made it to Blu-ray. And sometimes they stick with those. Others may decide to switch over to the new Blu-ray which has been the case with all of the James Bond films. Now the only exception I noticed was that Universal HD did broadcast On Her Majesty's Secret Service a while back and they clearly did not use the Blu-ray because the contrast was not blown out like it is on the BD. The source they used clearly was closer to the Ultimate Edition Lowry restored version from a few years earlier. It was cropped, but the contrast was correct and the colors slighty altered from the old Special Editon DVD which had the original colors. It looked great, but clearly was not the new Blu-ray.