Satellite/DTV HD channels with 720 are in progressive mode, which means the picture is already non interlaced that should result in smoother movement. Channels with 1080 are interlaced and it's up to either the receiver or the TV to de-interlace the video content, and as you can imagine some are more successful than others. I personally leave the de-interlacing to my Samsung FHD TV rather than the receiver, by setting the video output of the receiver to 1080i so that the TV takes over with amazing results, provided the bitrate is sufficient as well.
1280x720 is a smaller frame so it also comes to either the receiver's or the TV's scaler to give the optimal results when blowing up the resolution to 1920x1080. So it's a compromise for both situations, and this is where the bitrate plays its role, as well as each channel's mastering. I have seen a few full HD channels producing better overall quality with less bitrate compared to others. Note that most digital channels also use statistical multiplexing which means the bitrate is auto adjusted by the content shown in real time, and this is why average bitrate is a more reliable indication than real time.