There are a couple easy possibilities depending on server config.

1. Not using SNI, and all https requests just respond with the same cert. (Example, go to https://209.216.230.207/ and you'll get a certificate error. Go to the cert details and you'll see the common name is news.ycombinator.com).

2. http upgrades to https with a redirect to the hostname, not IP address. (Example, go to http://209.216.230.207/ and you get a 301 redirect to https://news.ycombinator.com)

Could be a number of ways for example a default TLS cert, or a default vhost redirect.

I actually had a job once a few years ago where I was asked to hide a web service from crawlers and so I did some of these things to ensure no info leaked about the real vhost.

I don't think op said that they had the correct host header?

Who says they did?