Landing Page URLs Inconsistent

In my nurture campaign, I created used landing pages for Contact Us and Thank You pages using Web Forms so that I only have to do it once. I simply copied the link URL for the Contact Us page and pasted it into my emails. All works fine.  Problem is that I was not getting my customized Thank You page as the follow up. I went to the Form in design studio that I use in the Contact Us page and went to specify the Thank You Page follow up.  In the pull-down menu, I did not see the URL of my custom Thank You Page. 

The URL of the Custom Page is http://pages.microcopydental.com/WFContactUs_WFYYYY-MMThankYouForm.html

In the drop down menu, the there were 2 choices.  I chose WF ContactUSForm-ThankYou.  It worked...

Why aren't the URLs the same. This could get confusing. If I chose the other option listed for the Thank You page, i get a generic page. When I chose this one, my page displayed... shouldn't they be the same?

Thanks.