Bonding something and defining a contact region is not the same thing. Bonding something is like welding it together. There will be no localized stresses at the interface. When you define a contact interface Creo puts springs in between those surfaces. It then iteratively increases the stiffness of the springs until the loading is equalized. As the springs get stiffer, the local contact stress increases.
Here is a great presentation with some contact interface examples:
