Comb generator vs. known EMC emitter - different measurements in shielded tent vs. 3 meter chamber
Here is a random arrangement I made with a generator and output measured in a transmission line with reflections to simulate an RF tent with reflections.
The peaks and valleys are due the waves adding and cancelling on certain frequencies that correspond to different wavelengths.

Ricean Fading

the dominant wave can be a phasor sum of two or more dominant signals, e.g. the line-of-sight, plus a ground reflection.
This combined signal is then mostly treated as a deterministic (fully predictable) process, and that the dominant wave can also be subject to shadow attenuation.
This is a popular assumption in the modelling of radio channels with building, ground and wall reflections

Recommendations.
Although a Faraday tent may be good for blocking signals going out ( egress) or in (ingress) , as you can see, standing waves due to reflections can add or cancel at specific wavelengths in your regon of measurements. A reflective tent lacks the RF absorbing materials common to expensive anechoic chambers.  (ferrite sprayed cones)

best test is outdoors in a valley with low background noise.