Say you have a steel knife cutting through wood. The strength of the steel in the knife means it breaks wood and not the other way around, but it does not guarantee that it will go all the way through. You'd need a considerable amount of force to slice a piece of wood with a knife.
Now let's apply that to the claws:
- When Wolverine cut that sink in half in X-Men: Origins he barely pressed that deeply into it.
- When he cuts into a car while riding away from the barn he doesn't get thrown off the bike but instead there is a clean cut along the side of the car.