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Okay, on to your question! The short answer is that the scoring algorithm is very complicated and it's not based on any kind of "average" of your performance or your difficulty level earned across the section. It's basically unlike any test you've taken before, so it really doesn't make logical sense.
First, think of the GMAT as a "where you end is what you get" test. You can lift your score to the 99th percentile but if, by the last question, you've dropped down to the 50th percentile, then your score will be 50th percentile. It won't be some score in between 99th and 50th. This is why you could lift your overall average difficulty level and yet get the same score (or an even lower one!).
So I'm going to speculate that this was at least part of what was going on. Go into your test results and look at the problem list. The right-most column shows your score trajectory throughout the section. Is that what happened - you lift high earlier on but then you dropped by the end of the section?
If so, then there are some things to learn about how to maintain a steadier performance across the entire section. A lot of times, what happens is this: you're doing well and get some hard ones right, and then get even harder ones, and then spend too much time, and then have to rush towards the end...and where you end is what you get. Better to let some of those really hard / long ones go in the middle so that you can maintain higher overall performance all the way to the end.
Assuming that I'm right
read this:
https://www.manhattanprep.com/gmat/blog ... -the-gmat/and this (with special attention to sections 4 and 5):
http://tinyurl.com/GMATTimeManagementand this series:
http://www.beatthegmat.com/mba/2015/09/ ... gmat-quantAside: I know you weren't working on verbal, but yay! It went up 5 points! Take it!