dB units: full spectrum versus ??

[cuibono]

This is a units question:

I'm using the Visual Analyser software's spectrum analyzer to watch some noise signals. From 20-20kHz, the graph is showing the a noise floor around -120 dBpp (is that a real unit?) - but there is a single number readout of the noise floor stating it is around -75dB (also doesn't seem like the proper unit).

Why does the software report a -75dB signal when they are all clearly at about -120 (dBFS?). I'm guessing there is some sort of integration function, some complicated math, and it all works out, but I would appreciate an explanation.

I understand decibels somewhat, but the units used don't seem correct. And can anyone easily explain the difference in the numbers?

I could note too, that the signal clipped at 0 dBpp on the spectrum analyzer graph, and the scope part showed clipping at 1Vpeak-peak (I'm not sure that is a valid use of units either...)

Thanks