Well.... it is not a great score since as you said yourself the test is not really adaptive.
We did our best to try to estimate but it fluctuates (almost every test does, even adaptive ones). Even GMAT has a 40 point error (due to stress, guessing, and topic tested).
The other element here is that GMAT Club questions are harder than what you would usually encounter on the GMAT (it takes longer on the GMAT to get to the harder questions). Here we start throwing them at you pretty much right away, so you get to them faster and don't waste as much time. As the result we had to adopt the scoring system to allow more questions to be missed (you got 12 wrong and still got a Q51).
The bottom line - you got a great score but take it with a grain of salt.
Founder of GMAT Club
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