fields_hierarchy

--values
---card_exp_date_month
-----und
-------4

That exists in form_state.
Unfortunately that hierarchy is followed by the form array too:
--(Some fieldsets)
----card_exp_date_month
------und
--------#default_value
---------4

The right hierarchy i think must be something like that:
--values
---field_name
----und
-------0
--------value
-------------4

and in form:
--field_name
----und
------0
--------value
---------#default_value
------------------------4