due to how forums/topics/posts are marked as read/unread in phpbb, the global threads spanning multiple forums circumvents the resource-saving methods phpbb employs and while it may seem like a glitch, it's the last unread time of that global thread's ghost/clone thread IN THAT FORUM since you last read it, etc.
ex:
http://forum.mafiascum.net/viewtopic.php?f=7&t=66545
http://forum.mafiascum.net/viewtopic.php?f=8&t=66545
same thread (66545)
different forum (7 vs 8) = different "last read/unread" timestamps
the tl;dr is that it COULD be done correctly via much, much, more programming, and doing things on a more granular level in the fundamental way that tool works, but phpbb (and indeed most forum softwares) use a sort of a shortcut which causes "bugs" when people use multiple features together in ways they weren't expressly designed for (which may not necessarily be a bad thing, given the nature of what usually constitutes a Global Announcement -- it's a Big Deal, and is commensurately treated as such, without regard to the normal being-marked-as-read confines by which mere peasant Normal Topics abide).
disclaimer: i really have only a fundamental understanding of this myself, here's a code-related topic i read once with more info on the general idea of this here, if you're interested further (you're probably not, unless you're a coder, and that's okay too):
http://stackoverflow.com/questions/2288 ... pics-posts
hope that explains it well enough, if i didn't.