Good questions! There are two main reasons why this might happen.
First, it might be the case that this particular passage is just really hard for you, for whatever reason. In that case, I'd see whether I could understand just the very basics of the big picture—no details—and then hopefully get the one main idea question right (and guess on the rest). (I would
not try to get deeper into the details to see whether that helps. If I'm not even getting the big picture, then I don't have the necessary context / framework to understand the details, so that's a waste of my time.)
I don't think this is what happened in this particular case, though, because you describe being able to understand the passage "easily" when you looked at it again after.
So that brings us to the second reason: Performance anxiety. When we're really stressed out, our brains go into fight-or-flight mode—literally, evolution taught us to focus entirely on how to flee from this situation and to expend zero brain energy on anything not related to fleeing. This can happen in all kinds of situations, not just life or death ones, and it definitely happens during standardized tests.
So while your brain is busy freaking out, it can't concentrate on what this sentence or paragraph or passages says about a topic that you're not actually interested in reading / learning about yourself—you just have to because it's on this annoying test.
While this is happening, people will tell me that they kept reading the same sentence or paragraph over and over but it was like their brain just wasn't taking it in at all. Or that they kept finding themselves thinking about something else and had to pull themselves back into this passage / problem. They also sometimes tell me that the test felt really strange—very unfamiliar and like they kept seeing things that they'd never seen before. Many people also start to have decision fatigue and do one of two (seemingly opposite things): Either they speed up and just start picking answers very quickly, like they don't really care anymore, or they find it impossible to make a decision at all and agonize forever over each answer choice / problem.
Afterwards, people report that they don't have a very good memory of the test or parts of the test—it seems like a blur and they can't recall any specific problems or content areas. If it's a practice test (so they can look at the material again after), they'll report that *now* the question or passage is fine / easy. They'll also report reviewing a problem they got wrong and thinking, "Wait, why did I pick
that answer??" and they can't remember at all what they were thinking when they were answering it for real during the test.
I know you already reported one of those things—the passage seemed easy after. Did you experience any of these other things?
If so, you're not alone. Most people experience at least some level of performance anxiety when taking this exam. You need some mindfulness training to help you manage the stress and stay focused during the exam.
This article talks about stress management in general and explains what mindfulness is; it also links to some free resources from UCLA.
https://www.manhattanprep.com/gmat/blog/2013/05/09/stressed-out-meditate-to-lower-your-anxiety-and-boost-your-gmat-score/There's also the 10% Happier program. It has a free trial that I thought was really good—it'll give you a sense of whether you like the program enough to pay for it. I've had several students who have actually paid and really liked it, so that's another option.
https://www.tenpercent.com/