The benefits of switchable attenuation for spectrum analyzer measurements

During measurements there may be certain spurs that do not have an obvious cause. Are they caused by limitations of the SA? Or are they present in the input signal?
An example is this two tone measurement of the input IIP3 of a mixer


The many spurs below -70dB are cause by bad shielding of the two signal generators. Without these connected the noise at about -100dB is without spurs
The SA automatically finds the peaks and calculates the input IP3 in two independent ways, the results should be equal but there is some difference.
Left IIP3 is calculatec at +9dB where right IIP3 is calculated at +7.7dB
But can we be sure the IIP3 of the mixer is indeed around +8dB?
The simplest way to know is the add attenuation before and after the mixer.
Attenuation after the mixer did not change anything (as it should) but -10dB attenuation before the mixer resulted in a very different picture.
The measured levels are increase by the level of attenuation to keep the displayed levels equal so the noise floor moves up about 10dB

The results (15dB improvement of IIP3)  is not entirely what was expected as every dB reduction of the input signal level should  increase the IIP3 with one dB.
There is still more to investigate and learn.