User can change web content status to Expired using one of many options on web content list (in control panel) and in web content edition page. But there is also a function that does almost the same. In schedule section of web content there is a checkbox saying "Never Auto-expire". If you unmark it you can set date for expiration. After that date you won't see that article but only message saying "This web content has expired". So user may think that those are the same functionalities. But they are not - when web content is after its expiration date its status doesn't change.
This is incoherent situation and may lead users to different confusing situations especially that it's not documented anywhere (in User Guide there is only one sentence and it says that "Allows to set the date when the content will expire" which is misleading).
The best solution will be to change this so after expiration date status is also changed. It also may lead to performance improvement because Asset Publisher is looking for articles with status Approved that are not after their Expiration date. So if you refresh page every second there is a query sent to database that checks Expiration Date with accuracy to one second. That means such results can not be reused by Mysql cache as the query is different every second. Also checking just one condition is faster than checking two conditions.