Are you using multiple tabs to browse the forums? If you dismiss in one that could explain why the number wasn’t updated in another.
Unread lists topics that you have entered and read sufficiently so that the forums have started tracking that topic for you. As such you get the confirmation that asks if you really don’t want to read these updates, and/or whether you would like to stop tracking them.
You can also tell the forums how you would like the tracking behavior to work, in your preferences under “Notifications” you have the option of setting the parameters for this. By default after 4 minutes in a topic, it will be set to “tracking” for you. You can of course change the level of tracking for each topic at the bottom of the topic - or when the timeline is available, this button below it.