The unsold thing is on average. They use the historic load factors for the flights and city pairs involved to allocate award seats based on how many seats in the past might have flown empty on that flight. The allocation could be zero for flights that run at near 100% loads.

This is also why you won't see very many open award seats on flights 5 or 10 months in the future. Maybe just 1 or 2 per flight. It becomes easier to predict as it gets closer to departure time.