User search problem

I have users that basically are numeric value.

If I try to search a user by login field:

1) if use "contain" (ex. 322) it return correctly that user

2) if use "is" the combo load the name and surname so it work.

if I search with method 1 and then switch to "is", by default are loaded the user where id=322. This problem is present using API. if I search (from API) login is 322 it try to return the user where ID=322 (and not login=322)