Alert Viewer severity filter count is not correct
The filter count numbers represented by the colored icons on the toolbar of the Alert Viewer, should show the total number of events, including all parent and child events in the list. However, when you select a filter, child events that match that filter but whose parents events do not, are showing in the list but not in the count number.
The filtering options in the Alert Viewer only work on top-level events, so if the other events that match the severity are children of a parent that does not have that severity, then they will not show up in the filter count.
This is a known issue and there is currently no workaround.