I'm not sure what the OP is getting at, because you're right, they indicate stud centers for 16" on-center. He might be talking about the 3-4-5 trick (pythagor

I'm not sure what the OP is getting at, because you're right, they indicate stud centers for 16" on-center. He might be talking about the 3-4-5 trick (pythagorean triples). But the only place I can think of where that is really useful (roofing) has other, better, tricks for figuring out lengths of hypotenuses.