The signal strength scale on TVs is just a relative scale; the scale will vary from brand to brand, and it isn't even the same on my different model Sony TVs.
The important thing is that you can use it for comparisons, like how weak can a signal be and still be received. Or you can use it for aiming an antenna while watching signal strength and signal quality as defined by SNR and uncorrected errors.
I used my signal level meter to calibrate the signal strength scale of my KDL32R400A:
In the screen shot channel 16 measured 57 on the Sony, which is equal to -2 dBmV which is equal to -51 dBm. Channel 16 on my report is listed as having a signal power of -29 dBm, so the signal at the tuner is 22 dB weaker than listed. The signal is weaker inside because of the building and tree loss. but it is made a little stronger because of the antenna gain (on UHF). The report assumes that a simple dipole antenna is used; that is the reference for a 0 dB NM signal.