Tagged items are very likely read by the user already, before they are tagged - so unless you mark them as unread after tagging they won't show up in the tagged item count. And then they also show up in the usual feed folders though they've already been dealt with. Wouldn't it make sense for the tag folders to show a count of total items?

Mr.Reader uses this approach, it's a lot more helpful IMO. If you must show unread count within tags it could be shown in as total items/unread items. I'd personally prefer just the total count showing.