Why is it if you post a global announcement.
Then say edit it for a update.
It does not appear as a new/unread post.
I would think that if you have a global announcement that it should appear as new, using the current timestamp of the edit date. Instead of using the original timestamp of the post.

Since a member can't reply to one, is this the reason why.

I think it should work that if the post is changed it goes to the top of the active topics list.