Hey Bilal,
This is incredibly common. I would stay away from Kaplan's CATs. They aren't "bad," but I just don't think they stack up next to the official CATs.
The only four CATs you should put faith in score-wise are the GMATPrep 1, 2, 3, and 4. I'd stay away from that old Powerprep software. It is simply far too outdated and inaccurate. The diagnostic in the OG15 is simply a pen-and-paper exam, and is not reflective of the actual exam's scoring algorithm. By all means do those questions and learn from them, but you should take the GMATPrep's (and most likely take them each twice) before you take your GMAT if you want to be in a strong place.
Beyond the official CATs, the other CATs I'd recommend are the Veritas Prep and the MGMAT.
MGMAT's Quant can sometimes be a bit harder than the actual GMAT, in my opinion, and can focus on some obscure rules, but they are indisputably excellent. The Veritas CATs are also good, but I am biased since I wrote some of the Verbal questions on them.
And if you're interested in a hypothetical score generator based on multiple test prep company's CATs, GMATClub has created this fun one:
https://gmatclub.com/gmat-score-calculator/
However, my honest opinion is I do not recommend my students take the GMAT until they have score 20 points HIGHER than their target score on at least two GMAC CATs and one private company's CAT.
So for example, if you wanted a 700, you aren't likely to be guaranteed to hit that unless you've scored a 720 or above on the GMATPrep 1 and 2, and a MGMAT CAT. Make sense? Be safe, not sorry.
Good luck!