My assumption would be that there's either multiple strains (which has been suggested many times that there's at least 2 or 3 main ones) or that people aren't developing immunity
But I guess the question is how long between tests, and did they have a negative test inbetween?
One thing from my hosptial is a fair amount of patients come back negative because the sample they took weren't properly done. Apparently the throat swabs need to go deeper down then typical ones so there's a tremendous amount of testing error (and the issue is more specifically user error not a faulty test)
But the the other issue is because of the nature of this there's no standard test, so you literally have hundreds of thousands if not millions of organizations world wide all utilizing a different process so how the freak do you compare data?
I know the institution I work for literally invented their own test so that's a perfect example.