Cristin,
There are several factors that go in to why these measurements are different. These include the software image rejection algorithm, the noise floor / sensitivity of the SA44B in the selected mode, and the effective video bandwidth and averaging method based on detector settings.
If your goal is noise spectral density measurements, the preferred instrument is the BB60C or SM200A. This is because the SA44B does not have hardware-based image rejection, so there is noise present from both sidebands (even if the signal from one sideband is rejected in software).
That being said, real-time mode, power average is a good way of measuring the noise energy (sum of energy at desired frequency and image frequency–subtract 3 dB typically for noise at desired frequency only).
For a lower noise floor, using sweep mode (image rejection ON), power, average, VBW <= 1/10 RBW, will use a more sensitive detector circuit and provide a lower noise floor than real-time mode.
