Legitimate reason is that they confuse one measurement for another when they are either different or presented differently. Earlier this claim was made that stereophile measurements showed 120 dB of attenuation while mine showed 80 to 90 dB. I pointed out that the stereophile showed the same as their "zero" reference as -40. So the differential was 80 dB. This happens fair bit because people just go by the punchline text in the measurements and not look in detail.
The bigger reason is that they don't want to believe the results. So they resort to talking points and other nonsense. They need to be reminded that I have tested around 1,200 products. If measurements were wrong, you would see a line mile long from manufacturers saying the measurements are wrong. We don't see this because it simply is not the case. While mistakes can happen from time to time, they simply are not routine. And at any rate, could easily be shown with identical setup and explanation which almost never happens.
Bottom line, listen to manufacturer providing own measurement with different outcome. This is something I welcome in every review. If this doesn't happen, then that is it. Folks complaining because they have websites/youtube channels doesn't count.