When I create an event and then select "Not Attending" once it's launched the counter in the sidebar is accurate but the Attendee count under the cover image is not correct (see attached ...)
Also, it might not hurt to reload the page after a user clicks Attending, Maybe, or Not Attending as currently nothing happens until the page is loaded by the user manually ... kind of confusing for them. My first though when testing was that clicking the button triggered a JavaScript error, which isn't the case of course, but left me wondering why the count / page had changed or updated.