Webarchive: Are the contents stored locally or not?

I was under the impression that importing a web page into DevonThink as a web archive actually stores the data locally. However, each and every time I open the archive, the contents of the page are reloaded again from the internet.

What exactly is a web archive then? How is it different from a URL?

Web archives are only useful if they don’t require cookies or a certain referrer, are not the result of posting a form and do not load dynamic content (e.g. scripts/PHP loading additional stuff like ads). In these tricky cases I’d recommend PDF documents.