That’s typically the result of users deleting accounts directly in the database. The functions that count users queries to the usermeta table to determine how many users are of a given type, when users do manual deletion they often forget to remove relational data left in the meta table, which in turn throws off the counts.
Images below are to show how what i’ve said above is true(though i’ll admit there’s always a chance this isn’t the problem you’re experiencing).
Two admins
Deleting a user record
Viewing the list of admins after user record deletion
If you want to check the installation for redundant user meta, you could find user IDs that don’t have a corresponding record in the users table in PhpMyAdmin using some SQL.
SELECT DISTINCT user_id FROM wp_usermeta WHERE NOT EXISTS(SELECT ID FROM wp_users WHERE ID = user_id)
If you’d like that query wrapped into some WP ready code, just ask.. 🙂