The Default Delphi Version property in Global Configuration / Main is used whenever the Project setting Compiler Version in Project / Settings / Main is empty.
To set the Default Delphi Version, do the following:
Go to the Global Configuration tree view and click the Main node.
Locate and click the Default Delphi Version property in the property inspector to the right.
Click on the drop-down button appearing to the right and select the Delphi Version