The difficulty level of your questions is not what determines your score -- it's how you perform, relative to that difficulty level, that matters. You could get a Q50 on a test full of medium-level questions if you got almost all of them right, and you could get a Q20 on a test full of hard questions if you got almost all of them wrong.
The test is adaptive, but it does not adapt completely predictably (since there are factors besides your performance that go into choosing your next question when you're in the middle of a test). So you can be doing very well on a test and still see many questions that are not challenging for you -- that's not something to worry about. But what will happen more often is that the test will seem challenging almost throughout, because if you perform at your level early on, the test will, most of the time, deliver you questions near your ability level. If you got a Q43 on a test that seemed easy to you, that usually suggests that you got some easy questions wrong early on. Getting easy questions wrong is the worst thing you can do on an adaptive test (very high-level test takers almost never get easy questions wrong, so as soon as you do that, the algorithm will think there's a very high probability you are not a high-level test taker). If the algorithm doesn't think you're a high-level test taker, it has no reason to give you high-level questions. You'd be able to recover from a couple of wrong answers on easy questions early in your test, but you'd have essentially no margin of error. If you continued at that point to get some medium level questions wrong, it would be hard to get to the high 40s.
It would be easy to confirm or refute that explanation by looking at an ESR -- just look at your performance in the first quarter of the test, and check the difficulty level graph to see how hard the questions were that you answered incorrectly. To avoid that happening in the future, you may want to relax a bit early in your test, and just be careful to ensure that you're getting questions below your level right -- spend an extra ten seconds per question early in the test confirming your answers make sense and that you're answering exactly what's being asked, when you know exactly how to solve a question (but don't do that when you have no idea how to solve -- you can see very hard questions early on too, and you don't want to waste time on a question you don't know how to answer). You should also confirm you have no gaps in your Quant foundation -- if you're weak in one topic, then on some tests you'll get easy questions wrong on that topic, and then you'll have trouble scoring well. If you always get questions below your level right early in a test, I think you'll find your scores are very stable, in the Q47 range (unless you improve your level further). If you sometimes make careless errors early on, and sometimes don't, you'll find your scores fluctuate a lot, because wrong answers on easy questions are so harmful to your score. Good luck!