OK4450, I personally like your third theory. Lack of knowledge/machine tool exposure.
Your other point about not knowing the preexisting condition of the threads is always a danger. I’ve changed oil on vehicles not my own where I’ve almost needed a breaker bar to get the plug out. Scares the bejeezeus out of me. That’s why nobody touches my drain plug but me. I think lack of proper training is a factor in these cases as well… and in some cases perhaps lack of integrity.