I understand it is because PB is monitoring every pilots hours individually on a weekly basis. If you are above the average of 73 hours a month - you are rested from the roster to bring you back in line.
I am told there is a memo in crew control about some new software FR have working that is managing the hours - so all pilots will be working 12 months of the year.
I suspect all Capts will get close to 900 hours.