Subclasses have a different relationship between interfaces and abstract superclasses. A subclass that imple ments an interface is saying simply that it "acts like" that specified by the interface. The class makes no statements however about fundamentally what it actually is. An actor implements a fiercesome alien from a distant planet in one movie and a fickle feline in another. But an actor is actually neither. Just because the actor portrayed a interplanetary alien, doesn't mean that the actor fundamentally possessed all the abilities of such an alien. All it says is that in so far the context in which the actor was utilized as the alien, the actor did implement all the necessary behaviors of the alien.
A subclass is fundamentally an example of its superclass. A subclass automatically contains all the behaviors of its superclass because it fundamentally is the superclass. The subclass doesn't have to implement the behaviors of its superclass, it already has them. An actor is a human and by that right, automatically possesses all that which makes a human: one head, two arms, 10 toes, etc. Note that this is true even if the abstract class has 100% abstract methods - it still enforces a strict taxonomical hierarchy.
implements is about behaving, extends is about being.