I do not wish to demean, but I don’t use jQuery. I’m just reading the documentation. This boils down to a Read The Fine Manual case.
http://api.jquery.com/jQuery.ajax/#jqXHR
jqXHR.done(function( data, textStatus, jqXHR ) {});
An alternative construct to the success callback option, refer to deferred.done() for implementation details.
Then a bit further down.
If json is specified, the response is parsed using jQuery.parseJSON before being passed, as an object, to the success handler. The parsed JSON object is made available through the responseJSON property of the jqXHR object.
This says you should be looking at the 3rd parameter’s respinseJSON member, not the first parameter’s members.