Over time those bigger numbers become more common too. Someone who can mentally do the type of problem I just did and get it right quickly likely have a ton of practice and will know quicker tricks, and be able to simplify it in a way.

Another part is they would be able to recognize a wrong answer more accurately as well. I didnt realize my answer was off by a lot until I put it in a calculator, but someone with more practice might know intuitively they were wrong.

I just don't consistently do this type of math, I used to be good at it in school but its become mostly irrelevant for me outside impressing someone a slight bit. It is helpful to have the ability to do things manually but it just rarely comes up.