When you select an object, there can be multiple identifiers for the element (instance, type, family, category, etcetera).
The Selection Resolution (on the Item Tools Ribbon Tab, Select & Search Ribbon Panel dropdown), gives you control what level you are looking at when selecting an object.
If you have the Selection Resolution set to Last Object (or anything other than First Object), try setting it to First Object and see how the GUID values are affected.
Lance Coffey
Technical Support Specialist