A subtle change appears to this dialog box if a library requires checkout.
Sometimes people in charge of your site will set up a library so that it requires you to always check out your files. How will you know this? A subtle change happens when you try to edit the file. You'll see that the option to edit the file is changed to Check Out and Edit.
This setup is common at many organizations. In fact, your company might have libraries set up this way, and you may have been checking out files without realizing it.