Signal "falling down screen" with low signal levels

I have had a problem for some time; I don’t know if it’s a fault or a feature.

I am measuring noise figure using the SA44. Typically I would have a signal of -90dBm and would compare it with a signal of -96dBm. I often set 1dB per division to get the best measurement.
However, sometimes I put a -90dBm signal into the SignalHound and the signal will start falling down the screen until it reaches the -130dB. I don’t know why this is.
Yesterday I set the analyser to 10dB/div and I found that if I had the top of the screen at -45dBm the “falling down” effect didn’t happen. If I set the top of the screen at -55dBm it did.

Any ideas as to what is going on and how to work around it (or if it’s a fault?).

best wishes,

Ed Long
Guildford England