Why lower bound() is mutable and upper bound not () in Java

Question

I'm trying to understand Java Generic so I ended up with not able to understand why lower bound type can be mutable and upper bound types not.

The example I have:

List<? extends String> upperBounded = new ArrayList();.
List<? super String> lowerBounded = new ArrayList();

upperBounded.add("A"); // Does not compile (capture of ? extends String)
lowerBounded.add("A"); // Compile just fine

So could someone explain me the reason that upper bounded types can not be mutable and lower bounded types can be?

Thank you!