As far as I'm aware, all of the official practice exams are built the same way, so I don't think one of their exams is easier than the others. But a lot of people do think that the official exam can feel easier at the highest scoring levels. (It's true in my own personal experience. I've gotten an 800 on the official practice exam multiple times but the highest I've ever gotten on the real thing is 780.
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The official practice exams have a separate question pool for each exam (contrast that with ours, which has one huge database that all of our exams draw from). The potential drawback to having a separate question pool is that the pool may end up being a little too shallow at the ends (low and high). So my guess is that this is true for the official practice exams and so it can be easier (depending on your strengths and weaknesses and the luck of the draw) to get a higher score.
But the difference you reported is a pretty big difference—I don't usually see that big of a difference between ours and theirs. You have been studying for the last two weeks, so part of the difference is probably that you did actually get better. And part of the difference may be what I described above.
I have a third potential hypothesis to add to the mix: How was your time management / executive decision making? When you mess up your timing on this exam, that can have a pretty serious impact on your score—and that can sometimes explain a 100 or even 200-point difference. Our test puts at least as much time pressure on you as the real thing does (and maybe a little bit more—I want you to be overprepared going into the real thing!), so part of the difference may be indicating a time management / decision-making issue.
The real test is more stressful than any of your practice tests because you know it counts this time, and that knowledge can *seriously* mess up your decision-making and time management. So if that was part of what was going on, then I'd want you to address that before you get into the real thing. Our test (in my opinion) does a better job than the official practice test of teasing out this particular issue / potential weakness.
Best guess, I'd say that your score has probably increased in the last two weeks, so you are probably in the upper 600s now (if not higher). I'd want you to take a second exam to consolidate that 700+ score to say that you're solidly up there—but you are moving in the right direction for sure!
Aside: Have you seen / been using this?
http://www.manhattanprep.com/gmat/blog/2016/08/19/everything-you-need-to-know-about-gmat-time-management-part-1-of-3/