Chinese companies do the same exact thing, if you honestly think you have ever bought a "Brand New" piece of mining hardware, you are to stupid to even be here
they all do it. Hell i would do it, and I am a pretty honest business person. However I, much like the Chinese, would at least clean it and ship what was ordered by my customers.
These things are manufactured and INSTANTLY put to work ( AKA testing) then when ordered by the reseller/retailers they to receive the machines and instantly put them to work (AKA secondary testing) then when you order a machine they pull it off the rack clean it replace the PSU and ship it out. The replacing of the PSU is not always something they do, However I do know the Chinese companies do replace the PSUs before they ship them out
Why do you think 2 of the same exact machines perform differently? They are manufactured the same exact way with the same exact pieces. yet 1 will hash better then the other. if they have all the same exact parts one would tend to think they would perform very closely to the other. But they dont because one was more rigorously "tested" than the other one