One of the key reasons for the 777/787 rated campaign is exactly the same reason as the A320 rated campaign - training capacity. The BA training system is running full speed and rated applicants get through the system in less than half the time, using half the resources that a non rated pilot does.
Looking at the bid results, a seniority of 3400-3500 where people are moving Airbus to Longhaul seems pretty much bang on 5 years in the company. The numbers are simply representative of the amount of recruitment that has gone on over the last 5 years.