In integrated mode, IIS App pools allow Any request URL to come in to the ASP.NET ISAPI, however, in classic mode, you would need a third-party ISAPI or the request will be sent directly to the page.
In integrated, the module gets checked FIRST before the actual request content.
SO:
Integrated Mode: http://www.yoursite.com/myfile.html first goes through your http modules and routes configured in http modules and global.asax (your Request.Url should have the URL above)
Classic Mode: http://www.yoursite.com/myfile.html checks to see if there is actually a file called myfile.html, and if not, then it goes to a 404 page. UNLESS, again, you have a custom URLRewrite module.
Hope this helps ya.