My Signal is 96 and SNR is about 18.6dB. Which I guess is good.
The OTA signals vary naturally because of constantly changing atmospheric conditions, levels of interference, reflections, even the wind blowing branches. It happens in analog too, but can't be perceived.
Unless you are extreemly close to the broadcast towers and have excellent sheilding any OTA STB will show wild signal flucuations, some stations more than others. My Zenith OTA STB shows one station near perfect and instantly drop down to bad and back. This is a multipath problem. Other stations will stay rather stable with just very small ups and downs.
Unlike OTA the satellite signal isn't prone to relfections from every direction, and the DBS frequencies aren't used by others terresterally so there isn't going to be much in the way of inteference.