I read the info a bit differently, Ian. I interpret it as, assuming the ground capacity is 50%, 50% of each reserved seating area will get a ticket from the ballot, and if you miss out you'll have priority the next week.
I'm not sure about any ballot for GA tickets (away team members have to be considered too) or if Reserved Seat members who miss out in their area can go in the GA ballot - there is a FAQ page on the Membership site for Reserved Seating, but not for General Admission. Here is a bit about the ballot:
For reserved seat and reserved bay members (including 3121 and Club XIII), opportunity to access tickets and attend match day will need to be done via a weekly ballot specific to each membership product. Given the size of our membership base, our priority will be to ensure as many members as possible have the opportunity to attend as many games as they wish in their usual seating areas.
am.ticketmaster.com