GMAT Prep

I am warming up with my GMAT preparation and gave the GMATPrep 1 as my diagnostic test. Turns out I scored 700 (48Q, 38V). 15 incorrect on the quant and 13 incorrect on the verbal part. I was more shocked than surprised, as I expected a score somewhere in the upper 500's considering this was a diagnostic test. I understand that this software is the most accurate indicator of test day performance, but I don't know what to make out of this score. How accurate is this thing?

FYI, I am not new to GMAT. I had given the test 2 years ago after prep-ing for roughly 1.5 months and scored a 680 (47Q, 35V) on the main test. I had never crossed 660 on my practice tests at that time. I had used up both the GMATPrep then but when giving the GMATPrep this time, I distinctly remember that while I could faintly remember the questions (like a couple of diagrams and formatting of one of the questions), I could not remember the actual questions or the answers or the methodology to get to the answers.

I don't know what to make out of this test. Please help!