List <? extends X> is a way of saying "This list can be assigned a list of X or X subtypes and nothing can be added to it". So you don't try to add a Double to an Integer List, a bit like in your in your example.
Also because you don't know what types are actually in there, just that they extend X, the only thing you can safely get out is a type X. The compiler can see this and throws a wobbly if you try anything else.