Why is a subclass in Java an instance of the super class but not assignable to it?

For a very basic example:

public class Human {
    //human stuff
}

public class Developer extends Human{
    //developer stuff
}

Why is Developer an instance of Human, but the class Developer is not assignable to Human?

Developer dev = new Developer();
System.out.println(dev instanceof Human); //true
System.out.println(Developer.class.isAssignableFrom(Human.class)); //false

But in addition to this: Human is assignable to Developer.

System.out.println(Human.class.isAssignableFrom(Developer.class)); //true

In normal English I would say that a Developer object is assignable from a Human class, not the other way around.