.com unread topics for moved topics

Hi, this is something I have noticed several times but never figured out the root cause. As this occurs on phpbb.com it's not like I can set up test cases here. What I noticed was this: I read every topic in GD, yet on index.php the GD forum icon still indicated unread posts. I went in and clicked "Mark all read" thinking it was a cache issue, still red. Next I went to phpBB Discussion forum and read all but one topic. When I read that topic, in phpBB Discussion, the GD forum indicator was reset. Weird, I said to myself. Then I wondered if perhaps that topic had been moved from GD to phpBB Discussion, and that I can't check on my own. Here is the topic:

http://www.phpbb.com/community/viewtopic.php?f=64&t=1801355

As mentioned, I have seen this behavior more than once, but only this morning did I consider moved topics being shown as unread in their original source forum.