You configure an RSS export, with articles exported from there. When an article is removed, the RSS export is not updated, and the article is still shown. Clearing rss cache fixes this.
This can be seen as a feature request, or as a bug - in single cases, clearing rss cache may be a good enough solution. On big sites it is tedious, especially when you frequently use automatic object unpublishing/expiry.
The rss cache is global per feed, and does not allow you to clear cache for a single object.