Specifying port 80 in URL gives different result than no port specified

I created and deployed app on Heroku. Now accessing the app via this URL: https://electricity-manager1.herokuapp.com/api/hello works as expected the result is;

{"express":"Hello From Express"}

But when I specify port in the URL like this:

https://electricity-manager1.herokuapp.com/api/hello:80

I get different result: the home page.

How is it different? I tried with port 443 but same result. I thought port 80 or 443 are the default ports when using http/https protocol?