No wonder the ut fandom thought during the first few years of its existence that Papyrus was dumb. People looked at him with Sans’s eyes and in Sans’s presence Papyrus, for some unknown reason, acts as if he knows less. Labradory, Snowdin made of ice-cream, Sun-related question etc. Tho without him he gives hints he knows enough, knows about lab, knows about snow, knows about cosmic things and finally has very smart books and doesn’t want his bro to know he actually is under great pressure while being the king (one of neutral endings). I think about maybe Pap initially holds some sort of naivety for his bro so he wouldn’t feel completely useless and only shouted at for something that wasn’t done. Maybe Pap likes things being explained in short manner (or he likes to give people a chance to ramble about their passions, such as asking Sans about something from space).
My tiny headcanon is that Pap has this as a habit (“Does a lightbulb have a spark of magic inside? Nyeh heh, of course I know how it works, there’s no magic inside but electricity!”). In scenarios where he meets alternative skelebros, this is what he would do to swap Pap or fell Sans if that was appropriate. That could also work as a method to get to know someone. Plus a method to hide his true self behind a slightly wrong impression. The impression that the ut fandom got. You all had been tricked by The Great Papyrus. And you’re lucky he isn’t villain.