"core center" is a term used by k2 only. Its not a real term for a mount position. The fact that you said true center could have been interpreted as center of the recomended point, not the true center of the ski. Next time, you want something mounted at a specific place, don't tell them where. Take a big, black sharpie and draw a line on the ski where you want them mounted and point.
Words can get confusing, a line saying where isn't. I don't have much of a case if its your words against their's.