I have a theory that the fields associated with the url parameter are not picking up the sources because it should go under Referrer Parameter, but I'm not sure what the difference is. Here's where I'm selecting URL Parameter:
The URL parameter option would be parameters in the querystring of the page you're currently on. The Referrer parameters would be for parameters in the page you just left.
So lets say you go to this url www.google.com?parameter1=testvalue and search for Marketo. The link on the page takes you to www.marketo.com?parameter1=marketovalue. You're now on Marketo's site, and the url parameters available are parameter1=marketovalue. The referrer values available would be parameter1=textvalue (from the google page)
Does that make sense?
URL is the URL of the page or landing page your form will be sitting in. Referrer is the URL of the page from which the lead was coming (the page displayed just before your landing page).
Most of the time, though, capturing referrer parameters is difficult as people will come from https sites, and if you own site is not protected as well, referrer info will not be visible.