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Al's picture

There's some information on Wikipedia about Noise Power, it's a function of bandwidth and temperature.  In other words it doesn't take into account other noise sources such as the thermal and shot noise of components in the dongle, noise from the USB bus, external interference and so on (it's a long list).

The power spectral density analysis that this software uses gives relative values for it's input data and some tuners may skew the levels with automatic gain control. Also remember that the decibel (dB) is a ratio, not an absolute value like dBm which is referenced to 1 milliwatt.