Lower bounded wildcard not checked against upper bounded type parameter

I wonder why does this piece of code compile successfully?

Source code:

abstract class A<K extends Number>
{
    public abstract <M> A<? super M> useMe(A<? super M> k);
}

Compiled successfully

How does it work and why does this compile? M is any type, so why it can be used?. Should it be: <M extends Number>? This will not compile:

abstract class A<K extends Number>
{
    public abstract <M> A<? super M> useMe(A<M> k);
}

Error message:

type argument M is not within bounds of type variable K where M, K are type variables: M extends Object declared in method useMe(A) K extends Number declared in class A

What is the difference?