Writing about this topic because I messed up last 2 days before my exam because of this myth, So for context I had given all 6 of the GMATprep Practice tests and I was consistently getting 720-740, So I googled to see how much my actual score might vary from these tests.
Some blogs were positive saying they shouldn't vary much others were pretty negative talking about how GMAT has evolved and the Practice tests were old and that I can expect to get 40 points less in my official exam and hearing this I kind of panicked because that would mean I will get 680-700 and that was way less than what I wanted. But anyway, I kept these feelings behind me and went for the exam and would you believe it I got 740(Q49,V42).
So, to conclude if you're reading this post 2 days before your exam like me please don't panic. The official exam is pretty similar to GMATprep tests.
Best of Luck