What happens is that this stuff isn't simple.

One problem is that "the same model" isn't guaranteed to be. The people who made the kit years ago aren't making it now, or making the ARF of it now. Or two guys who put together what actually are similar models do things differently. Or two workers in the ARF plant do things differently.

A year ago we were looking closely at two identical ARFs from the same retailer. One was at least a couple of years old, the other brand new. It was amazing how many things were different from the factory. And darned if the two CGs were amazingly different considering both guys claimed to have balanced his plane exactly where the factory recommended but with "just a bit of nose down to be safer".