The Universal Worklist uses cache mechanism. Unless items are retrieved from backend, there is no way of knowing ahead the number of items. Hence, only after each tab is clicked upon, will the relevant items be retrieved from the backend system, stored in the cache and the number of read, unread and new items be computed and displayed on the tab.
This behavior can be controlled.
The value of "keepItemCountUpdated" property in </NavigationNode> tag in uwl.standard xml configuration regulates that.
<NavigationNode name="notification2" view="NotificationsView" referenceGroup="" visible="yes" keepItemCountUpdated="no" referenceBundle="notifications">
As shown in the above xml snippet, since the value of "KeepItemCountUpdated" is set to "no", only when a tab is clicked will the releavant items be retrived in the cache from the backend system and respectively their number will be shown.
If you would like nonetheless to have the numbers of tasks updated and displayed on the tabs without clicking, you can do this as described in this online help.
However, be aware that this might impact negatively the performance.