Any kind of online test is inaccurate. Test questions are finite points used to extrapolate an entire curve/line that has an infinite number of points. Esp. the multiple choice questions that have weighted scores, ie. "If you answer yes to any question in group A, give yourself 1 point, 2 point for any question in group B..." That's why the higher up you go in education, the more lengthy the tests are, the less "multiple choice" they are.
A written test where a scorer has to sit down and read what you write and determine if you grasp the subject sufficiently is like a very large number of points on a segment of the line that represents the real body of knowledge. It's way better than multiple choice but it's still not complete.
For self-discovery tests, it's even trickier because you can undoubtedly study for the test. Once it's in your subconscious mind, it's impossible to get that knowledge out. The more Enneagram tests you take, the more contaminated you are because you're not stupid, your subconscious mind learns what answers are desirable I'd say if you never knew about the Enneagram, never read any forum, and never had any desire/preference for what type you are, the first test probably has some meaning. Once you get on any forum, or read any book, that's it. Further tests are useless.
For example "I like to have my own way" A) always, B) usually, C) often, D) seldom, E) never... You won't answer this question the same way after going through the forum a few times, you can't unlearn it.
The best way is to read and reflect. It's like asking your spouse: "Are you a loving person?", while you have a whole life time to learn about that person. The book is right in front of you.