How does HR Notifications use the PS Query Recipient Population method?

I am setting up HR Notifications for our users for the first time and I'm trying to understand the Recipient Population using a PS Query,

I've read PeopleBooks and my interpretation of the use of the HR Notification > Notification Recipient Setup > Select Recipient Popluation Method page is that I should be able to use a PS Query to return all the emplids of the population that I wish to send a notification to.

However, on the HR Notification > HR Notification page using To box prompt  when I select my PS Query Recipient Type and Click search the search results contain emplids that are not returned by the query.