The numbers displayed by ATSC set top boxes and software with tuner cards indicate some arbitrary signal reading - but are not normally percentages. I would not consider it useful to compare the readings from two different boxes (or PC software with computer tuner cards). Comparing the numbers from the same boxes would be useful.
The ATSC broadcasts represent a continuous bit stream of about 19.39 mbps (mega bits per second). In that transmission stream there are a lot of PID's. You can consider a PID to be information or data ... there's some standard ones every transmission needs to have ... and then some that would depend on what is being encoded and sent (unique to each station).
Your set top box / ATSC receiver may detect an ATSC transmission ... the PID with the basic info on what's being transmitted is usually quite small - representing something like 18 kilobits per second of data - and can be easily decoded as it is pretty minimal info. (As you can see, it's a very small percentage of the entire transmission.)
You may notice your tuner stopping on channels that have ATSC transmissions ... but those channels may later not show up in your list of available channels. That would likely be due to too not being able to lock onto the ATSC signal and/or having too high of an error rate. Your box may have detected the basic info but was otherwise unable to really decode reliably the whole stream.
The meters (numbers) that the STB displays may be related to the RF levels ... or they may be related to the bit error rates, the latter IMO would be more desirable. For example: you could have a whopping RF signal, but if you have a bad multi-path problem it could cause enough data errors that your viewing experience would be unsatisfactory.
(Multi-path means the signal from the station's transmitter may actually take more than one path in getting to you ... bouncing off terrain, objects, etc. At 300,000 meters per second - the speed of light - the signals you receive may arrive at slightly different times. In analogue TV these fractions of a second differences often cause ghosting, etc. but in the digital domain you get data errors, e.g., drop outs, etc.)
In decoding ATSC, when you tune to a sub-channel with your STB (like our 22.3) ... your STB is looking at a PID - in our case it would be 0x0040 (that means hex 40). PID 40 tells more about what's in the 22.3 sub-channel, including what PID has the video information (0x0041) and what PID's have the audio information. In our case - we have just a primary AC3 Dolby Digital audio feed and that's on PID 0x0044. (If there was a secondary audio channel, like SAP, it would be on 0x0045, etc.)
Your STB then receives the continuous stream of bits from the station's transmitter. Those bits are organized into packets of 188 bytes (characters) each. Each packet has a PID identifier; your STB would discard the packets it is not interested in (like those for other sub-channels).
A typical HD 16:9 format program encoded around 15.0 mbps will consume over 75% of the bandwidth of the entire channel for the HD video content. (In our case - that would be packets with PID 0x0041.) In relation, the audio PID for that channel would only be 192 kbps - that's 0.192 mbps - just a few percent of the total transmission bandwidth.
It would be normal for now and then to get a packet or a few packets in a row that are not properly received. In many cases - you may not even see or hear those errors. But if the error rate increases ... you may see pixilated video, artifacts, large square areas not being updated, audio dropouts, etc.
If you're having reception problems and the signal meter on your STB stays pretty constant ... the meter may be showing you RF signal levels. OTOH, if you're seeing the meter bounce around ... it could be either RF or error rates. Pretty hard to tell.
Also note: many STB do not necessarily update that signal meter as fast as you may like ... some are only once a second or once every other second.
In the end, the best and really only way to see what kind of signal you are receiving ... is not with your STB, but with a device called a spectrum analyzer. This device looks like an oscilloscope - and graphically displays the energy coming down from your antenna (or cable system coax). Here we can easily see what the actual signal level is (typically measured in a decibel or dB level) ... as well as see if there's anything interfering with it (as in nearby transmissions, etc.)
BTW - up in the Springfield area ... using different boxes and antennas ... with the new WTIC-DT I can get anything from a detection of the channel but inability to tune it ... to the PSIP information that provides info on the channel (61.1) to a pretty decent (error free) signal.
Sorry for being so verbose - hope this helps!