You are correct, here is Cisco's explanation (copied from a 7920 doc):
The recommended DSCP or PHB marking for voice control signaling traffic has been changed from DSCP 26 (PHB AF31) to DSCP 24 (PHB CS3). A marking migration is planned within Cisco to reflect this change; however, many products (including the Cisco 7920 phone) still mark signaling traffic as DSCP 26 (PHB AF31). Therefore, in the interim, Cisco recommends that both PHB AF31 and PHB CS3 markings be used for access to call signaling queues.