Since the fix of MySQL bug http://bugs.mysql.com/36742 GRANT, CREATE USER and DROP USER all lowercase the host name portion of user name argument before going to the mysql.user table, so ensuring that the actually stored host name part is always lowercase.
The mysql_system_tables_data.sql template used by mysql_install_db doesn't take into account yet that the system hostname may use upper case characters in its set hostname
How to repeat:
On the shell do "hostname FOOBAR"
then do a fresh server installation using mysql_install_db
check the mysql.user table to see that entries with the
uppercase FOOBAR were created instead of the expected lowercase foobar