From the ESRs, I can see what happened, but I can't say why it happened. So you'll need to use information you have about your strengths and weaknesses, and about your diagnostic scores, to judge which of my speculations below are true and which are not:
I'm not sure how your diagnostic test scores broke down. I'd ignore any prep company scores, but if your Q score on official diagnostics was in the Q49-Q50 range, your test day Quant performance is completely normal. If, however, you were scoring Q51 on official diagnostic tests, then a Q49 is significantly lower than what you're currently capable of. Since your Verbal scores must also be lower than your ability level (if you were scoring 700-760), if you had been getting Q51 scores, it sounds like you're simply underperforming on test day. Then you'd want to consider why that might be happening. There could be all kinds of reasons -- test day anxiety, fatigue, distractions in the test environment, or just bad luck (with content balance or with guesses) -- and I don't have enough information to guess what the reason might be. So if you are normally a Q51 test taker, then the main thing to work out is why you're underperforming on test day, and nothing I write below really matters. But if you're a Q49-Q50 test taker, then you're not significantly underperforming on test day, and then what I write below is relevant.
- in Quant, your performance looks very normal for a Q49-Q50 level test taker - there are no obvious areas you need to focus on. It looks like you have a solid foundation in every topic, and you're performing the way high-level test takers perform; you're getting almost everything right besides the extremely hard questions. Even though your score didn't change, you performed better on the second test. It appears on your first test, you may have been stubborn about a hard question or two in the middle of the test, and then ran out of time near the end. On the second test, your pacing was much better, and if you can continue to pace yourself that well, you'll have a better chance at a Q50 in the future.
- it's in Verbal where I'm sure your score on test day differs most from your diagnostic test scores. Your Verbal ESRs are a bit unusual, as I'll explain.
- in SC, your performance changed dramatically from the first test to the second. Not only were you much more accurate at SC on the second test, you were also much faster at it. I'm not sure if the first test was just a bad day, or if you made genuine progress at SC, but if you can repeat your SC performance from the second test in the future, you'll have a shot at V40+ scores. That SC improvement is solely responsible for the four point change in your overall Verbal score;
- in CR and RC, your performance was similar on the two tests. In both tests, somewhere in the middle, your performance suffered a lot. Not only did you get several questions wrong, but you also spent a lot of time doing so. That time investment didn't turn out to hurt you much, because your accuracy was unusually good near the end even though you were short on time. But the wrong answers there hurt you a lot.
One unusual feature of your tests, especially the second one, is that your Verbal questions were much easier than normal. That can happen to anyone, and there's nothing you can do about it -- the test does not adapt nearly as predictably as most prep books claim. Some tests are easier than you expect, based on your level and performance, and some tests are harder. That doesn't affect people's scores, but when a test is easier than normal, you need more right answers to get a good score, and when it's harder than normal, you need fewer right answers. On these tests (especially the second one), your hit rate was good, but because your questions were never very hard, each wrong answer hurt your score quite a bit.
If you take another real test, it's not likely you'll have an easier-than-expected Verbal section (most of the time, people seem to report seeing a harder-than-expected Verbal section these days). So you might have a completely different experience on your next test. But it would definitely be worthwhile working out why your score was lower than expected on these tests, since that will help you no matter what happens on the next one. The problem is, from just a couple of ESRs, I have no way to guess what happened. These are a few possibilities, but some of them will definitely be wrong:
- your timing and accuracy data is what I'd expect if a test taker loses focus mid-test when facing lots of long CR and RC passages. If you think that might have happened to you, you might want to mentally reset one third of the way through Verbal (just take a minute to stare at the wall and clear your head). You might also think about section order options, if you're losing focus in Verbal;
- It's possible, especially if you've mostly practiced from high-level or prep company questions, that you're "outsmarting yourself". Your questions were mostly easy and medium level, and if you sometimes thought "this answer is too obvious, it can't be right", and chose something else (which might be the right way to think about very hard questions), then you might have gotten questions wrong that you should get right. The obvious answer will be the right answer on easy questions;
- it's also possible, again especially if you've been practicing from hard problems or unofficial problems, that you have some gaps that prove to be important on lower level problems. There might be logical issues tested in easy and medium level CR questions, say, that you aren't sensitive enough to. Then it could be true that you do fairly well on harder questions (which is what you might have seen on your diagnostics) but not as well on easier ones (which is what you saw on test day).
There are other possible explanations as well. Long story short, it's primarily because you got easier RC and CR questions wrong on your real test that your score was lower than normal. If you can figure out why that might have happened and address it, you should be in a great position to score in your normal 700-760 range on your next test. But just from a couple of ESRs, I can't say precisely what you should be doing, so hopefully you can combine the above with information you have from your prep and work out a good plan. Good luck!