Miears stated that they were under Ransom's command. In my mind there are two possible explaination. Both Pettigrew and Ranson were from North Carolina and since his diary was written after the fact from memory He may have become confused. This is the most likely explaination to me.
OR because of Ransom arriving, pretty much unattached, at a time when Pettigrew had been wounded and captured Ransom may have been put in temporary command of the Brigade for a very short time that was not recorded in the "Official" orders and records. That was not uncommon either until things could get sorted back out after the battle.
As all re-enactors will attest to, when involved in a large scale event, that even the "official" reports are only "snapshots" of things that were seen or heard about in a very confined enviroment of that persons experiences. No General could ever have seen the entirity of the Battle he commanded. and that the average soldiers experienced the Battles with tunnelvision, not seeing what might have been going on just 50 yards or less to either side of them.