That’s the wrong question 
The question we need to be asking is “how can my noise level be an implausibly low level”?
One common reason is that the audio that you are analyzing contains “absolute silence”. “RMS” is a kind of “average” measurement, so any absolute silences will push down the measured noise level.
Try using the “Sound Finder” effect to see where the impossibly low level occurs (See: Sound Finder - Audacity Manual)
Use these settings (below). Any parts that are NOT labelled are below -100 dB.
