Although sample collections of explanatory variables where u1 is close to u3 (markers in the white area near the straight line in Fig. 4(a)) are meaningful, sample sets where u1 is far from u3 are worthless to the accuracy of the response surface.
This is because CP 1 is located near CP 3 in the entire structure, and therefore, u1 would be close to u3 in general.
The regression surface does not have to express data sets where u1 is far from u3.