The post "failure" condition fires after the "always" condition (and I number of others), but before the "success" condition and "aborted" conditions.  This means that if there is a failure in the "always" condition steps, the "failure" condition steps will fire.  But if there is a failure in the "success" condition steps, the "failure" condition steps will not fire. 

This would make sense if the the "failure" condition only fired if the stage steps failed but never fired if one of the post condition steps failed.  But that is not the case.