Hi! Yeah, the algorithm is...weird.
The GMAT doesn't base your score on number correct or percentage correct.
The algorithm is calculating your score after every single question—so it's basically a running scoreline throughout (that's how it decides what question to give you next, incidentally). It doesn't wait until the end and only then add up or average whatever you did to find your score.
So you can basically think of your score in each section as this: Where you end is what you get. You could lift to 99th percentile for two-thirds of the test, but if you tank for the last 10 questions...where you end will be your score. It won't matter that you got a bunch of really hard questions right earlier.
Well...that's not entirely true. It will matter in the sense that you'll be dropping from, say, 99th percentile to, say, 65th percentile. If you had lifted to only, say, 75th percentile at your peak, then you would have dropped to maybe 50th percentile, because you weren't starting as high. But it isn't the case that the test is like, "Oh, you got the last 10 questions wrong, but now we'll also average in all those earlier ones that you got right, so your score will be somewhere in between those two levels..." Nope. Where you end is really what you get.
That's why we show the scoring trajectory (the final column on the right) as part of your Problem List data after the test is over. Go take a look at that trajectory for tests 2 and 3—you'll see that even though you did earn harder questions on the third test, your improved performance didn't carry through all the way to the end of that test. You finished at the same level, so your score stayed the same.
Here's the good news: You earned harder questions. You did that by answering harder questions correctly—more than you did on your prior test. Some of the category averages are even better than your first test. So you're learning how to do more / moving in the right direction. But you've got some test-taking stuff to make better in order to make sure that your performance carries all the way through the end of the section.
What happened at the end? Were you running out of mental steam? Did you have some added time pressure and have to go faster and make more mistakes? Did you get unlucky and see a few more things at the end that were in your areas of weakness? Something else? Some combination of all of the above?
When you know what caused the drop towards the end, you'll know what to target to make it better next time.
One thing that a lot of people overlook is mental stamina—when you decide to keep going on certain hard problems, you are using up mental stamina that it might better to use later, at the end of the section. This is especially true for your second section (eg, if you did Q, then V). You have to think about how to spend your mental energy on Q not just to make it through Q and finish Q strong—but also for V, since that's coming second and you can't adequately recover mental energy between the two sections.