The GMAT is an extremely competitive, international test that challenges even the brightest of students. I'm a GMAT tutor and Harvard grad with honors (class of '02) who has earned perfect scores on the SAT and ACT. I've taken the GMAT 4 times now, with a high of 770 (47Q /48V / 99%) and a low of 700 (46Q / 40V / 89%), and I'm still trying for my perfect 800.
Here are some of the many reasons why it's almost impossible to guarantee an 800:
-The Essay (AWA) and the IR section. These don't even count toward your 200-800 composite score, but they take up the entire first hour of the test! And most B-school programs consider your scores on these sections too, so you should still try your best (if not quite as hard as on Quant and Verbal).
-The Pacing (1.5 minutes per question on verbal, 2 minutes per question on Quant). The GMAT requires you to move quickly and confidently from question to question, with no time to waste.
-The adaptive nature of the GMAT and its scoring algorithm. You keep getting questions right, and the questions keep getting harder until they are *extremely* challenging. You also can't skip questions or go back, of course.
-Experimental questions. There are 23 experimental ("pretest") questions on the GMAT, but you don't know which ones they are. They don't count toward your score at all. If you get lucky, then most (or all!) of your incorrect answers will be on experimental questions. If you get unlucky, then you will perform well on the experimental questions (or spend a lot of time on them), which of course doesn't help you at all.
Think about this crazy fact: you could get all 23 of those experimental questions wrong, and all the other questions right, and still earn a perfect 800 score with only 74% correct (67/90 = 74% correct), if you happen to have incredible luck. Conversely, if you have terrible luck, then you could get 23 questions wrong, all of which are counted questions, and this might earn you a GMAT score of about 550-600, even though you got the exact same percent of questions correct!
-Other random factors such as lack of sleep and test-taking fatigue, abnormally tough RC/CR/SC questions / Quant questions on esoteric topics, wrong guesses on 50/50 questions, test center issues, etc.
Even those of us on GMAT Club who have achieved perfect GMAT scores will admit that they could never earn a perfect 800 every time. There is always an element of chance involved. That's why the GMAC had to institute an 8-test lifetime limit...students were taking it again and again, and trying to get lucky.