Hi Twitter,
Since you're allowed to retake the GMAT after 16 days, GMAC has to make sure that you face an entirely new set of questions. However, each of those new questions has to go through a rigorous 'testing process' before they're certified as "fair" and added to the pool of questions that you might face. The only practical way to test out those new questions is to place them in the actual GMAT, so that real Test Takers can respond to them and GMAC can gather the necessary statistical data to properly evaluate each question. The scored questions that you face on Test Day were once 'experimental' questions for someone else (and those questions did not count towards their scores); in that same way, your Exam will also include a series of experimental questions that will not count (and those questions will show up on someone else's Official GMAT later on).
The GMAC questions writers are pretty good at their jobs, and there's a set of 'standards' that all questions are measured against - in that way, experimental questions will look just like standard GMAT questions that count. As such, you won't be able to tell whether you're looking at an experimental question or not.
GMAT assassins aren't born, they're made,
Rich