Accuracy MGMAT CAT

Wow, great start, hellfridge! A 690 when you're only shooting for 600+? Awesome.

To answer your question, Manhattan GMAT's tests aren't perfectly accurate, since no test-prep firm can perfectly copy the exact style of the actual GMAT, but they're usually pretty close. If you've heavily used MGMAT materials, then it's conceivable that your score is biased upwards, but a 690 on an MGMAT test looks awfully good to me, especially with such a solid quant/verbal balance.

It's always possible to miss a ton of questions on the GMAT quant section and still do really really well on the test. The GMAT scoring algorithm basically gives you a score based on which questions you miss, not necessarily on how many questions you miss. So you can miss a crapload of questions and do wonderfully on quant, as long as you only miss harder questions, and get the easier ones right.

I hope this helps. Keep up the nice work!