Hi Øyvind,
It looks like what you are experiencing is actually expected behaviour: those events from August that are showing up are still current in September because they do not end until November or December.
So is your objective essentially to show only events that started in the month being viewed? This might make a worthy feature request and you are more than welcome to post one – this lets other members of the community comment and show their support for the idea, which can be useful when we prioritize new features and functionality 🙂
Thanks!