Why are not all stylesheets in mytheme.info being loaded?
the answer is in the drupal_get_css doco page:


  Themes may replace module-defined CSS files by adding a stylesheet with the same filename. For example, themes/bartik/system-menus.css would replace modules/system/system-menus.css. This allows themes to override complete CSS files, rather than specific selectors, when necessary.


If you look at the source code at the bottom of the page, the array that contains the css has the basename of the file as the key. So this is an intentional feature.

You will just have to keep the renamed file.

Hope this helps. :)