Such is the nature of ATSC digital television. There's enough error correction to repair the data down to a certain level, but when the signal drops below that threshold, bad things happen.
Looks to me like signal level and signal quality are backwards. A 100% reading on signal quality indicates there are no errors receiving the signal and should give you a lot of margin for error. On the other hand if the 51% is the signal quality and not the signal level then .... what harshness said.
I recall that EyeTV (a Mac computer TV receiver) had some screwy readings like that at one time. It appeared to have signal level and signal quality backwards.