When adding an Employee to the list of owners ("Owners" tab) for a service item in IT Shop, this is not updating the group owner or the service item owner, although it appears that way in IT Shop, as it shows as "Group owners".
When adding owners this way, the process is actually adding the UID_Person for that Employee to the existing role, the Product owner. These are the Employees listed under "Assigned". Thus, wherever the role itself is assigned, the Employees will be assigned that role.
In the image below, the service item has Product owner, TP\TREVORP:
The query is UID_Person in (select UID_Person from PersonInAERole where UID_AERole = '%UID_OrgRuler%').
UID_OrgRuler is the product owner for the service item. This is a role, and thus can be assigned to multiple Employees, just like any other role.
A service item can have only one owner, which is the role. If the desired result is to keep one owner per service item, then individual roles for each user, per service item would be required.