A listserver is not Usenet. Usenet is an entirely different system than the listserver. Although some people receive their usenet postings through their e-mail, most people do not. You should not presume that the way you experience a newsgroup is the same for everyone, or even for most people. Unlike email which is sent directly to your mail box the news resides on the news server of your service provider.
News is also copied from one machine to another.
It is entirely possible to see a response before you see the original question. When you want to read the news you make a connection to the news server. In some cases the posts will be sent to your machine and you read them from your local machine. If that is how your newsreader works every post will take up space on your machine until you delete it. In other cases you read the news directly off the news server.