I've asked about this before and I was told it is due to a difference in auditing tickets. As I understand it, CO and now UA, prefer to audit the reservation and the ticket prior to posting the miles. Other airlines will post the miles immediately and do auditing later and adjust as necessary.

As I understand it, if it is a GDS issued ticket (i.e., non UA directly) they need to wait for the ticket to be processed by ARC/IATA. Once the file arrives, they can reconcile and post the proper miles/PQD.

Just a difference in philosophy.