I can only speculate as to why the delta wing builder you used as your example chose the spot he did for mounting his servos. I can tell you, however, when I build a plane with the servos on the wing, my placement is dictated by the length of the servo lead. I do, however place my servo as close to the center as I can. In some cases I have to use a servo wire extension to get the placement I want. In most cases it is mostly pilot/builder preference.