I was thinking of one of the final episodes when they reveal that Kevin literally made up a number to fix accounting mistakes. I think they called it a "keleven"? Also, it could have just been Angela being Angela, but they seem to indicate that he has trouble with basic math a lot, lol.
Edit: Also, didn't Kevin apply to the warehouse and Michael gave him a job in accounting?
Kevin starts out the series as a meek introvert but later they change his character to be borderline mentally challenged. Kelly got a character change as well as the series progressed