Jason from Lark Rise Lower School has sent in a well explained answer.

The rule for giving another set of numbers is to put odd numbers at the top in ascending order and the even numbers in descending order.

Josh from Ampthill, UK has quite a different way of doing it.

In the top three boxes there is $1$, $2$, $3$. In the bottom three there is $6$, $5$, $4$, so each of the three pairs of numbers adds up to seven. (That is; $1$&$6$, $2$&$5$, $3$&$4$.)

Another solution from Sonya from Rocky Elementary School, Canada:

And another from Year 6 at Brookfield School, UK: