Ammon B. answered 03/15/19
Elite-trained GMAT Tutor (780)
I took the GMAT twice. The first time, I scored the exact same as I did on both practice tests. That was not good news, however, because I took the practice tests early on in my study. I attribute that to test day anxiety. You have control over a lot of things when you're studying. When I took my practice tests, I just sat down and took them. But on test day you have to fill out forms and you are being shepherded around and they're taking your fingerprint and when you come back in from your break you're being examined to make sure you're the same person. On top of that, you're in a completely different setting that you probably haven't seen before, so I think it's common for people to loose a little bit of their edge because of test anxiety.
Luckily, the second time I took it, I was at the same test center and I'd already been through the routine, so the new environment shock didn't affect me. I ended up scoring a bit better, so I guess I was either lucky or in the zone on that day.
If you can figure out a way to mitigate test anxiety then I think you'll be fine. And congrats on the practice tests!