Perhaps some think they see a difference because there just has to be one.
I Used AVIA and the built in calibration of my LG TV. In my case, I was prepared for there to be a difference, and was pleasantly surprised when the difference was so minimal as to virtually not exist. Sometimes it's the end result that matters the most and numbers do not tell the story. I understand Dish does not send a full 1080I signal, and OTA does. (Or 720 depending on network) But it just does not translate into a worse picture, on my 48" TV. As mentioned by others, the TV's abilities play a role also.
Years ago on a Mitsubishi forum using calibration the numbers would indicate to not use any of the artificial features of the projection TV. Yet most on that forum found the picture to look better with a couple of them on. Calibration showed it distorted the picture. Perhaps, but to our eyes it improved the picture. I learned from that it's really what it looks like in the end, not what the numbers say only.