A hint as to the reason is in the name of the tests.
test_default_value_is_the_same_object is there to show you that when you ask for hash[:some_value_that_doesnt_exist_yet], by default, you get back the default value you specified -- which is the same object every time. By modifying that object, you modify it for every nonexistent key. Modifying hash[:one] also modifies hash[:two].
test_default_value_with_block shows the construction of a Hash using a block, which will be used to provide a new value for each key. When you do it like that, the values for hash[:one] and hash[:two] are distinct.
