Maybe you can also help me understand what is different about the way the Rigol is performing the operation shown in that video?
You have selected math where is level1 - level2 = level (in your case level is in dBuV but also same if you use dBm level only numbers are different but principle same)
And your SA do it and it do it right.
But have you ever thinked what you see in your example video where SA is Rigol.
Now; look it carefully and think what you see. Is it possible that -20dBm - -20dBm is 0dBm. NO! But as you can estimate this happen in this Rigol video.
Now I ask why you select Math Power diff where result is power. (level difference where result is level) You know that 1kg - 0.5kg and result unit is kg, in this case 0.5kg. 1kg - 1kg result is 0kg.
But if you calculate example 0dBuV - 0dBuV it is 0uV and now I ask how many dBuV it is. Or example 0dBm - 0dBm result is 0mW but really not 0dBm but -infinite dBm
Think again what you see in Rigol video.
If there is -20dBm - -20dBm result is same as -30dBm - -30dBm in both cases difference is 0dB! and so on.
The answer is the levels difference deciBels and not the difference in level. dB is a ratio but dBuV and dBm is the signal level (relative to a reference of 1uV and 1mW).
Look carefully next image (yes my SSA is different but principle same)
Look these traces and relations... how it is calculating. Now you see difference in dB (look what math was now in use)
And this is same principle what you see in Rigol diplay....