Quite often there is a misunderstanding between displaying of a value and sorting upon the same. ApEx reports are giving you a possibility to show one value and sort upon a different value, which you even may not want to show to the user.
As always, I created a small example in my demo application containing a full explanation and a link to the corresponding thread in the forum:
http://htmldb.oracle.com/pls/otn/f?p=31517:153
It is easy to set it up and it shows the power of ApEx once again - it's simplicity.