Its deterministic behavior, lets understand how -
What is Given
Box1 - Box2 = 9 ----------- 1
Box3 - Box4 = 14 ----------- 2
Box1 + Box3 = 12 ----------- 3
Box2 + Box4 = 2 ------------ 4
Now Eq1 + Eq4
Box1 + Box4 = 11 --- 5
Eq3 - Eq2
Box1 + Box4 = -2 ---6
Now we can see Box1 + Box4 in Eq5 and Eq6 are giving two different values which is not possible. Hence its deterministic behavior.