There isn't much reason to trust scores from company tests. It's true that almost no companies emulate the real scoring algorithm, but that's not the main reason to distrust company test scores. The questions on company tests will overemphasize certain skills, and underemphasize others, as compared with the real GMAT. A company that makes its hard questions hard by making them very long and complicated is not correctly replicating hard questions on the real GMAT. Someone who is blazing fast at algebra will be overly rewarded on those tests; someone who is not, but who has excellent conceptual reasoning skills, will get too low a score on that company's test, but will do very well on the real GMAT. Of course people who are fast at algebra are often good at GMAT math too, so company tests often correlate fairly well with GMAT scores, but that's only in the aggregate. Individual test takers will often do much better or much worse on the real test than on a specific company's test.
You should sometimes be using tests to assess your level, since that's the only reliable way to know if it's a good idea to sign up for the real thing. But you should only put any stock in the scores from official tests, and you should bear in mind that luck plays a small role in determining your test score, so you should expect some natural fluctuation from test to test -- at your level, seeing discrepancies of 40 points from one test to another would not be too unusual, though larger swings on official tests would usually suggest you were performing worse on one test for a reason unrelated to your ability (fatigue, distraction, etc).