In a collection, the "content expiration time" and "maximum number of content elements" are only applicable to stories manually added to the collection. They do not apply to stories added via tag.
Also, the expiration time only works for stories added to the collection AFTER the expiration time is set, not to the stories already in the collection when the expiration time was set.
It would be great if the collection settings applied to all stories inside the collection, no matter how or when they were added.