Why does the warning 'Owner does not belong to Team Area' appear for some project area users but not others in a workitem whose filed-against category is inherited from the project area?

We have a project area containing a number of team areas associated with categories and one category not associated with a team area i.e. shows as inherited from the project area. All users are members of the project area as well as being members of certain teams. In a work item, as each category is selected the list of users in the owned-by field changes to the members of the associated team area as expected, except when the inherited project area category is chosen, in which case only about half the project area users are listed. The project area was originally created with no team areas and users were added, then sometime later teams were added along with more users in the project area and in the teams.