I dont know about AA but this can be very complicated, sometimes heavy crewing is required by union agreement, sometimes by legal requirements, but this is not just governed by sector length but factors such as number of sectors in the duty, start time, rest time before duty, are they acclimatised to local time etc. etc.
Sometimes other factors come into play, for example it could be that they then stay overnight in Manchester for 20 hours wake up jet-lagged and then have to fly back into a head wind taking an hour longer than the Easrbound journey with the usual tailwind and this return triggers the extra man.
It is a minefield to sort out sometimes!