The issues is "fare buckets." There might be only 2 seats left at the cheaper fare bucket.
I thought this at the first beginning, but booking the tickets for adults, no matter is 4/6/8 people, the price will always be $840 for each. but as long as I add children, no matter is 1adult+1kid or 2adult+1kid or other combination, adult's ticket price will be over $1000. really don't understand.