Actually, that is the shortlink version of the permalink. WordPress automatically generates shortlinks with every post.
If you visit that page, and view the source code, in the
<head> section, you will find `<link
rel='shortlink' href='http://www.planetahuevo.es/?p=2367' />`
Both links are actually the permalink. The plugin utilizes the shortlink for two main reasons, 1. To save characters in the Tweet and 2. To seamlessly integrate with other plugins that alter the shortlink (ie: WP Bit.ly - which modifies WordPress' shortlink to a Bit.ly link).
But, the plugin also utilizes the Tweet button's 'counturl' parameter (From the Twitter API: counturl = The URL to which your shared URL resolves to). This way, the shortlink is displayed, but the permalink is actually being credited for the Tweet. This is extremely beneficial, if you have a user that would rather copy and paste the URL into their own shortener and not use the button. The tweet resulting from this will theoretically still be counted.