That was a good learning experience.
1) If you create a GitHub repository for a personal GitHub Pages site…
https://github.com/user/user.github.io
2) And then configure that repository with a custom domain name…
3) And the published personal GitHub Pages site has a subdirectory named “foo”…
4) You will have a conflict if you have a repository named “foo”
https://github.com/user/foo
So, in your case, we could have renamed the droppin-dimes directory to dropping-dimes and it would have worked fine (no conflict).