The issues is "fare buckets." There might be only 2 seats left at the cheaper fare bucket. With one reservation, all passengers must be booked in the same fare bucket. Since only two seats (or maybe 3) are available in the lower fare bucket, they put all four of you into the higher fare bucket. When booking more than one person, always check 1, 2, 3, 4 pax up to the total number you need.

So best is to book two passengers into each of 2 reservations. Best is probably to book 1 ADT and 1 CHD in each of the reservations. Or else change your date so you find a date with 4 seats at the lower fare.