Really don't understand this situation, could any insider (agency) explain why buying 4 tickets, ticket price per person is more than buying 2 people only.

I was checking price on the China south airline online booking website, for the same flights/schedule, if I choose booking 2 adults only, each ticket is $840, but when booking 2 adults+2 children, for adults, each ticket's price will jump to $1029 and children is 75% of $1029.

why I can't get the $840 ticket for 2 adults+2 children?