Looks like phpmyadmin's username and password are stored elsewhere ( probably in a custom-defined config file ) in WAMP or there is some additional hashing or ... involved in the process.
So to change currently used default 'config'-file-based password you can browse "<host>/phpmyadmin/user_password.php
" using your browser. You'll be prompted to enter your mysql credentials and then you can use the displayed form to change the stored password for the user you logged into previously.