The industry seems to classify CMS as mostly belonging to purely Web content, whereas Alfresco has its roots in document management which is possibly why there's confusion.
In general, ECM systems are more geared towards processing and collaboration of content and tend to be deployed to solve different problems and perform different tasks to a web CMS. As an example, very few web CMS support document transformation and workflow, whereas these would be expected in an ECM.
Because Alfresco is built upon the ECM principles, as well as supporting "traditional" web CMS technologies, you're getting the best of both worlds!
Mike