Hi, this is actually a known limitation in the low band noise floor adjustment.
The factory calibration process ensure the power reading is accurate for all range, and therefore some noise floor discontinuity happens, this is normal in all spectrum analyzers of all brands, although in this case we recognize is higher value than ideally expected.
We are working in a new firmware to be available shortly which will flat the noise floor significantly and still keep calibrated amplitude reading wideband.
Cheers
RF Explorer Team