Counting unread items is HARD
I mean, if google can get it wrong, I have an excuse, right?
See how at the same time you see:
At the left: "All items (8)"
At the top-center: "8 new items"
In the tree: 8 items
In the list: 14 new items
Each of those numbers can get out of sync with each other if you do something the "wrong" way.
But what the heck, it seems as of r678 urssus does it right. The list of unread posts even updates when you have it open adding the new posts in the right places without disturbing you, and keeping the right numbers everywhere, AFAICS!
So here, in uRSSUs:
13 unread articles in the "Unread Articles" item
13 in "All Feeds"
13 adding each folder
13 in the article list
Sadly, this is all post-0.2.11 release.