How login is possible, if I deny login page via .htaccess with allow ip

You have denied the access to /wp-admin, however the user can still attempt to login via wp-login:

http://example.com/wp-login.php

Therefore, you should also deny wp-login.php:

<Files "wp-login.php">
    Order allow,deny
    deny from all
    allow from 123.123.123.123 // Your IP address
</Files>