The D4000 is a tuner/downconverter that is not optimized for noise floor flatness and variation, unlike our RTSA products where we optimize using internal adjustable attenuation. The noise floor variation is caused by gain and insertion loss variations in the front-end components, such as amplifiers and switches. Consequently, the noise floor of each D4000 unit will be different.
It is important to note that spectrum analyzer measurements can vary depending on the input level, and their accuracy is only specified within a range of input levels (-50 dBm to 0 dBm). As a result, there may be discrepancies between the measurements we take and those that you take. All of these uncertainties can accumulate over time.