weird behavior

[eluser]runrun[/eluser]
I create a sub folder in controllers, call admin, in admin folder I create controller called home.php.

Code:

application
  controllers
    main.php
    account.php
    login.php
    admin
      home.php

The weird:

I go to http://mysite.com/admin it directed me back to http://mysite.com

But 2 days later, I try again http://mysite.com/admin it takes me to http://mysite.com/admin/home

Why isn't it taking me back to main page like before ?