You'd have thought that a number was something easily defined. However, there are cases in everyday life that don't seem to follow the rules. My number plate, for example, consists entirely of letters. Similary, the PIN for online usage of my library may contain only letters, if I choose.
In the former case, the terminology probably came into existence when licence plates were only serial numbers, with letters being added with the growth in number of cars. For the latter, I can only think that the people documenting the system were thinking of the ATM PIN analogy, rather than the more usual (and correct) password.