Quoted from DCFAN:
When going back to the main screen the number will still be listed as green and then I click it again and it only has the posts I had just viewed.
Looks like possibly an IE and Firefox issue as I see it there but not in Chrome
I think the behavior he's talking about is replicated by:
1. Click on a green topic button and it takes you to the new posts since you last visited the thread as you would expect.
2. Click the back button on your browser and you notice that the topic button is still green.
Now If at this point you click the green topic button a second time and it again takes you to the new post but now if you click the back button on the browser the topic is now grey.
I also note the same behavior is present when after clicking on the green button instead of clicking the back button on the browser you click the forum links within the thread page you are viewing.