This has to do with the SA-series software-based image/spur rejection algorithm. Behind the scenes, it is injecting LO high side, making a sweep, then injecting low side, making a sweep, and picking the lower measurement. For this reason, the SA44B is not recommended for broadband measurements (such as broadband noise).
If you need to make a noise measurement with the image/spur rejection enabled, set your VBW to 1/10 the RBW. You will need to compensate for energy at the image frequencies if your noise bandwidth is >42 MHz. Subtract 3 dB from the measurement for these situations.
The alternative is a BB60C or an SM200A, which have excellent hardware-based image rejection and much lower spurious. These will provide good noise measurements without any tricks.