Yeah, I mean, I personally tend to do my “learning” in Normal/Heroic before even entering Mythic, but I wouldn’t expect a new player to have that. I get to pull on near 2 decades or experience to be able to project how things are likely to scale up, or just look at mobs/spell names and intuitively know patterns (e.g. that pack has an abomination, it probably will hit hard, Stimulate Resistance is going to be annoying if it’s not interrupted, etc.). There’s some things, like certain things in the Manastorms encounter that don’t even happen until a certain key level, so that’s also anomalies that can surprise players suddenly.
There’s also transferring knowledge from quest mobs to be able to tell “Ok that one’s going to hurt, this one probably needs to be burst down, this one could be annoying if it’s not stunned”, and so on. Dragons breathe fire, cleave, and have a tail sweep was a meme a decade ago, the other coin of that meme was “This is a different dragon, because it breathes PURPLE FIRE”. I’ve literally fought 3 versions of Hakkar, you put me in front of a Hakkar, and I’m going to have a rough idea what’s up without looking at a Dungeon Journal, guide, or video. Someone whose first expansion is Shadowlands cannot be expected to know that.
There’s also things like UI to consider. Someone with base UI, or who can’t read logs/death reports properly, has a much worse ability to diagnose mistakes/errors. Some people’s UI’s break on certain vehicles/posses bars. Or sometimes sheer RNG affects learning, take Kul’tharok. When he explodes at lower levels, you may have killed him a couple of times and never been picked for Draw Soul. Or Xav, you could have always been picked to go down, and then suddenly you’re the sole player up top. It’s unlikely, but theoretically possible (and actually would be kinda funny)
Also, in theory someone really good could change how a player learns. Let’s say you’re in a dungeon run, and you’re instantly interrupting the important cast that a mob does, because you know the mob will do it after the 3rd cast of this other spell. Someone else in that run might literally never notice that spell exists, because they had a 0.25 second window to notice X spell was being cast before you interrupted it. And it’s not like seeing a brief glimpse of a spell name lets them know what the spell is, and why it’s important (unless it’s a boss and it’s in the dungeon journal)