What you've likely discovered is that whatismyipaddress.com causes your browser to query a large number of other domains that don't appear in your URL bar. For example, I see all of the following domains queried when I visit whatismyipaddress.com:
fusebox.fm
omappapi.com
onesignal.com
whatismyipaddress.com
whatismyipaddress.info
I'd guess that the IP address detection is being done by a domain other than whatismyipaddress.com and you would need to add that to your domain list for the IP address detection to work.
I actually covered a similar use case in this thread.