jclimacus said:
At the time, I asked myself, what's the difference between those that see, but accept without objecting, and the majority who are sound asleep, who no doubt took it down in their notes without batting an eye?

I couldn't see a difference, and I still can't.
Good question and I think there's a risk assessment that is occasionally done by some people in certain contexts due to previous 'aversion' therapy or for other reasons.
Here is where the (so-called) attentional defect of 'distractibility' can be both advantage and disadvantage for a student, OSIT. Imagine a classroom full of students, intent on taking notes during a professor's lecture. At some point in the lecture, the lesson veers off any recognizable track of sensibility. After a moment or two of hearing gibberish unrelated to the main point, thesis or theme of the presentation, the student looks up from his notes, looks around at others and notices most of the others are still intently taking notes without missing a beat.
What is one reaction for the 'distractible' student unaware of a ponerization process yet feeling what may be its effects? A bit of pain in the head, a reflex action of wanting to put your hands over your ears to block out all further words, an intense frustration with being one of only a few people who are reacting like this and a strong desire to leave the room for some fresh air?
I've had this type of reaction and have read stories of others with similar situations and reactions. Part of the problem with the vulnerable students seems to be not knowing how much concentration the material is worth. If I focus so much that I lose my peripheral awareness (and thus lose 'distractibility), then I lose some ability to loop something said back to previous statements in order to catch where the b.s. starts, though I may not catch everything--including my own. :)
Some people seem to be in a state of full concentration for no particular reason. So much so that they don't easily catch that a statement like "there is no such thing as truth" is complete b.s. without having to prove it. Sure the grammar and logic structure of the statement is correct, but so what?
Sometimes objection might be withheld as a trade-off to get the h*ll out of that class, that subject, that grade, that group or whatever, as fast as possible in order to get into a calmer state.