"haha you said “PIN number”"

this is like, fake wit that fails to consider what language is doing. people don't screw up "can you give me your SSN" so the redundancy of "PIN number" is probably not an inability to process acronyms — and by the way "acronym" has come to refer to all initialisms, so don't even.

I think the prevalence of "PIN number" is because the situations where someone would be asked for it are also situations where someone would be likely to either have or be around the writing instrument called a pen, and "give me your PIN" sounds like "give me your pen", which creates a momentary confusion in the brain, so "give me your PIN number" is additional clarification to say "give me your p(e/i)n, by which I mean the number one and not the writing one"

philosopher Catharine Diehl commented a likewise intuitive explanation of “ATM machine”:

In my case, I think it has two steps 1) I think of ‘ATM’ as autonomous — I don’t think of what the letters stand for. But then I think of it as a type of thing and the type it is of is machine. So ‘ATM machine.’ Like ‘money machine’, ‘green machine.’ It’s something about losing the salience of the original and then the pull of the grammatical category. I would explain ‘PIN number’ similarly. Tbh I barely remember the ‘n’ there stand for ‘number,’ and I forget what the ‘p’ and the ‘i’ stand for. But I certainly know how abbreviations work in general.