This is a difficult question to answer. My first question is, what antenna distance are you using? Is it the same distance in both cases (certified test and pre-compliance test)? If not, there is a 20 log (d2/d1) dB correction to apply.
Obviously your chamber is not anechoic, which will change results, and VSWR on your antenna and amplifier can change things. I can see these stacking to produce maybe a 6-8 dB variation, but 20 dB seems a bit excessive.
Do you have two of the antennas? If so, you could calibrate your system in position using a signal generator instead of your DUT…
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