Hi Gustafe,
As has been said, no one, except for some folks at GMAC, truly knows how the GMAT algorithm works, so you will not be able to get a definitive answer to your question.
That said, I see two factors that may have held down the level difficulty of the questions that you saw and your score. One is that, as you mentioned, you missed the first question. Once you missed that one, you had to climb back up the difficulty range just to get to medium. The second factor is that your missed questions are distributed pretty widely throughout the quant section. As a result, you didn't get long streaks of correct answers that would have driven you to and kept you at the highest difficulty levels. Rather, your streaks of correct answers were broken up by missed questions. Also, it is likely that, by missing question 19, then bouncing back, then missing questions 23 and 24 in sequence, you kept the difficulty level down for a fairly long stretch. All of these factors contributed to your scoring Q49, which, by the way, it a pretty normal score for a quant section with 25 correct and 6 incorrect.
The good news is that, if you can score Q49, you are right where you need to be to score Q50. Just continue to analyze your practice exams and improve your weak areas. Also, analyze the questions you missed on this test to see what you could have done differently at your current level of quant knowledge that would have resulted in your getting a few more correct.
If you need any advice regarding your study plan, feel free to reach out.