The RTA has a 61-band graphic display and I can watch the noise floor portrayed on the display change in real time as I move the power cord around. This doesn't take an act of genius or any real training to observe the display and I assume that as it goes down, so does the background electrical noise level.
The Behringer has a noise floor of -113 dB and I was observing a noise floor of ~-100 dB, so it was within the Behringer's claimed range.