Thanks for that clarification. Yes, the implementation is fairly logical.
You will find limitation to surface assignments when you start using transparencies in animations, for instance. Individually assigned surfaces ignore the transparency setting during the animation. Only the overall assignment will take on transparency in that case. Something similar happens when you output to 3D PDF when the part takes on any one of the many random colors assigned to a single part. These are all things you learn slowly and turn out to be a real let-down when you counted on them working -as expected-.
Since half my work is in technical presentations, learning Creo's limitations during my off-hours is quite important.