I recently took an official practice test using the college board subject test book and scored a 740 (meh.) Does the practice test predict what score you are getting on the real test?
I usually do better on test day than on practice tests. Though I never took a practice test, I looked at my Blue Book practice test for Math II after taking the actual test and doubt I’d have gotten an 800 on the practice. I’d probably have gotten closer to what you got.
Don’t worry, just make sure you nail the timing and you’ll have a solid chance to get an 800. 