Newbie here; how can domains know what my ISP is?

Hello NextDNS users, 

I have recently gotten into privacy a lot more and started using NextDNS. I have set it up correctly, I think. The reason why I think I did is because I get a 'running' result to the 'nextdns status' query.

I have incidentally went on a What-is-my-IP-address kind of site and, to my surprise, learned that it knew what ISP I'm using. On top of that, they are reporting a IP address that is roughly correct, which is perplexing me. However, I'm not sure whose IP that is: mine or mine ISP. 

I have checked the settings on my.nextdns.io and they are set to hide my IP address.

A little help clarifying this would be nice. Thanks