So the default values are being picked from where .. is what we are trying to find out if I understand correctly . For which if you can share the screenshot or let me know exactly which place the custom field is created would be more helpful .Also you can right click and find out the webdynpro component for the same let me know that as well /.
Generally there is also a meta data concept for changing the data fields at item level .
PLease share the details and I shall check if I am able to help.