A variation configured in a site definition won't be resolved when you trigger the variation with an extension. For example, in /modules/extended-templating-kit/config/sites/default/variations configure a variation named print and try to trigger it with http://localhost:8080/magnoliaAuthor/demo-project.print.

If you configure the same variation in a page template definition it gets resolved. Also, if you trigger it with a channel name it is resolved, for example http://localhost:8080/magnoliaAuthor/demo-project.html?mgnlChannel=print