Domain Mapping Failed.

Hello,
I am having issues with my self-hosted WordPress site.
On 1/14/2016 I purchased a domain name from Godaddy, I then put said domain on my Gostgator server space.
On the same day, 1/14/2016, I mapped the purchased domain to WordPress, paying the $13 + $5 registration.
On 1/21/2016 I received an email stating the following:

During an automated check we noticed that you paid to register example.com(actual domain name removed), but it was not successful because this domain is already registered.

Normally it would not be possible to pay for a registered domain, but a temporary glitch allowed it to occur. I have refunded your payment.

If you own this domain with GoDaddy and want to use it on WordPress.com you can map it to us:
https://en.support.wordpress.com/domains/map-existing-domain/

I followed all steps in registering the domain, linking said domain to the host and “mapping” to WordPress. I purchased separate services (not registering domain with WordPress or Hostgator, ect) in case I change my mind, I have ownership of each item and can move it to a different service.

The question here is: Why did the domain “mapping” with WordPress fail, when I have ownership and mapped it correctly?