This code/plan can change...
http://cgit.freedesktop.org/packagekit/tree/backends/conary/conaryEnums.py
How define the group of a apps own.
Im have develop a algorith what do this
app1 has this categories: ['GTK', 'Graphics', 'Photography', 'Viewer']
it mapped on a dictionary with:
GTK its ignored check the py file. for seen what its ignored
group_graphics: 2 category exist ( Graphics, Photography) >>>> app1 show on Graphics group on Packagekit
group_accesories: 1 category exist ("viewer")
if exist a tie/equal pick the first.
so you need your the XML cache updated