Why do avatars have different height than original size?

Hello,

I uploaded an avatar that has a height of 25px but when I look at my webpage the avatar has a height of 23px. There is no css on the image so I am not sure what is causing this. Here is the code for the output below:
<?php echo get_wp_user_avatar($user_id, 130); ?></a>

You can also see in action at http://www.watchthefeed.com

Can you help me with this?

Thank you very much

https://wordpress.org/plugins/wp-user-avatar/