Hi,
As Bren and others explained, manufacturers play with these numbers a lot, to enhance the alleged performance claims.
There's another way to think of it or describe it. You can say, for example, "the noise is 80 dB down," which means that the noise floor (analog tape hiss plus thermal noise from transistors) is essentially inaudible if the signal level is at 0 dB. Or you could phrase it this way: "Noise measured -80 dB," which is saying the noise was 80 dB below the signal level, which is great since noise that far "down" is considered essentially inaudible in the presence of music programming.
Going back to analog days, it's amazing how much tape hiss and grunge we'd put up with. In fact, some misguided fans of vinyl actually prefer noise (groove swish + tape hiss) along with their signals. They do so because they don't understand some of the peculiar psycho-acoustic effects of noise when it accompanies music. But I'll save further explanations of that for a newletter!
Regards,
