The Consumer Reports reliability ratings come from their annual reader survey.
I've filled out the form for my cars, and it's not an exact science. For instance it might ask if you've had an engine problem but might not ask if it was a blown head gasket or a broken fan belt. For some problems they do ask for more details, but they don't for others. But I don't think there is a bias to the survey, it is just limited because people will only take so much time filling it out.
Then Musk wrote this blog with all kinds of data in which he seemed to tear the NY Times review to shreads (he also said the review caused Tesla's stock value to lose $100 million)...