First To Kathy's John (Or was it John's Kathy) Excellent post you are correct 100%
Now to LadyYatch
TV-Fool does not know about the big signal eating tree next to your site or sometimes even the building in the way... This is the reason the signal strength predictions can be 'Off' or rather one of the reasons there are others including your antenna.
Finally The station I'm watching now.. for some reason if I tune my SENSAR PRO to the proper carrier (35) and peak it.. THe signal strength on the TV goes DOWN.. WHY? I'm guessing signal to noise.. a noise source nearby gets stronger "Faking" out the Sensar pro but not the TV.