The GMAT is adaptive, meaning that if you get a question right, the next question will be a little harder. Harder questions are worth more points, so it's perfectly possible that you might get the same number of questions right but a different score. Your score depends more on the level of difficulty of those questions you got right and the placement of the questions you got wrong. In fact, you can get a great score and be getting every other question wrong towards the end of the test!

Hope this clears it up.

Thanks for the information Jim. My new aim is to get consecutively first 20 questions right.