S meter on CB radios are NOT Calibrated.
When I fix one I set the S meter to about S7 for a S 9 signal -73 dBm.
The reason I do this is because 90% of the customers listen to relatively strong signals from close up radios.
No Complaints. Nice to see the needle move but not always peg.
Again most customers use CB for business applications (which they should not, according to Govt.) And they use relatively low power compared to the previous conversations. Basic communication,
S meters are notoriously inaccurate but useful. I have a measuring receiver that reads good dBm of Rx signal but I bet very few people have one.
Almost ALL S meters are inaccurate, The best you can do is set it to ONE S meter value, the others will be off.
Almost ALL Bird meters read RMS, some do peak, so comparing music or voice is not meaningful, unless you put in a measured one or two tone audio signal into the mic input of the transmitter.
Some better AM transmitters will have a very low carrier and really large side bands, some will not work without a big carrier.
I have a radio that generates an "AM" signal using only USB and a vestigial carrier. No one can tell the difference without a SA.