Some authors will wait until they have completed a story before they begin posting it.
Some will get several chapters ready before they start posting and continue writing. The problem here, something can come along that can put the author behind. For example, an author can get sick and not feel like writing. The authors editor could have computer problems and not be able to get the chapter back in a timely fashion. These are a couple of things that I've seen happen.
For myself as a reader, depending upon how long the wait is, I may go back and re-read what I've already read to get back into the story. One story that I read was last update back in January and the next chapter will be coming out soon as I beta read for the author.