This has something to do with how your server redirects to the default page. You can see that you aren't specifying the default page in the url (index.php). This causes the form to not submit properly because there is no action specified.
You can probably fix the problem by adding action="index.php" to your <form> tag.
That is probably why it seems inconsistent to you, sometimes you visit with the page specified and other times you are just browsing to the directory and the web server is displaying the page.