Sorry, I don't think I understood your reply.
(What's is the difference between learning something on purpose or not?)
It's not about forgetting where you got the info from, it's forgetting that others (other brainholes besides your own) don't/might not know it.
It's just a characteristic of human(?) psychology, we all have it (it's not applicable/the bias doesn't kick in constantly, but it's there, you can't really function without it).
Information asymmetry isn't that intuitive to us, the languages/communication itself (memetics) heavily implies that - you just can't always explain everything from scratch, from inventing a form of communication to everything in the universe, etc just to tell someone to eg boil some water.
A basic example of this might be a child opening a tin box with cookies pictured on it only to find a sewing kit, then another child walking into the room, not seeing the opened box they don't know it doesn't contain cookies so they check. The first child usually thinks that the other one is stupid af bcs "it is known that there are no cookies in that box, duh, everyone knows that" (even forgetting that it's a reasonable assumption that there should be cookies in it). Children up to 4 (or 7?) can really struggle with that but can comprehend if explained to them (sooner or later we usually do learn it & try our best to compensate for it).
More nuanced cases are ofc when it gets tricky to correct the bias (you just don't have the data or the time to account for all of it, perhaps even a cultural hurdle as ppl might take offence to you over-explaining something they think they already know), eg at work giving tasks to someone less/differently experienced than you.
I think it's a subset of wiki/Curse_of_knowledge, but I'm no psychology expert so I'm not sure (might be classified differently, wiki/Cognitive_bias).