Hmm, Apache and IIS are engines on which a site is run off of, simply what controls what is being sent and received. Both Microsoft's IIS and Apache default to index.html, index.php, index.asp, index.aspx, etc as meaning that this is the one page you can find a way to get to the other pages within a website.
A webmaster could set this to be different if they wish, but most don't as it could cause compatibility problems if something is upgraded, and may cause issues with the end user (The one accessing the website).