I understand that clicking on a notification or if you go into your email the email notification disappears, for example. This is different. Mine disappear with no interaction.
1) I can be on chrome, for example, and hear a mail notification and see the icon pop-up in the notification area. And that same icon sometimes immediately disappears just as soon as it appears. I never have to go into the notifications for it to disappear.
2) I can leave my phone down for hours. On the AOD I can see I have a calendar reminder, email, and a Facebook notification (for example). Without ever opening the phone, I will look at the AOD and now only see the Facebook notification.
Those are just two examples of them disappearing.