It's just a theory, but I think some mathematicians intentionally 'unlearn' or 'forget' arithmetic when they start to understand what math is about because people who don't understand math will ask them if they add and multiply all day. Theoretical mathematics is an entirely different beast than what most people think it is, so mathematicians get a lot of dumb questions about it. When someone asks if they add and subtract all day, if they say 'I can't even do arithmetic', people may be more inclined to rethink that stereotype.
My wife is one of those 'math people' that struggles with simple arithmetic. She has calculators and excel for things like that. But for her it wasn't a case of unlearning it. She struggled in school with arithmetic and didn't start to excel until they started introducing variables and all of the other aspects that separate math from arithmetic.
A good example, but you should have thrown in an analogy to the general population thinking anyone who does anything with electronics is automatically a microsoft windows admin/personal trainer with special focus on virus removal.
