Columbia Historic Preservation Society

The dimensions of each component need to be controlled. Design tolerances - the degree to which each piece part is dimensionally identical to its siblings in a manufacturing lot, and to the original design dimensions - must be maintained. This also relates to how each component mates to the parts it interacts with.
And components that are not intended to directly mutual-effected or must never inadvertently touch each other. And it should go without saying that there can't be any foreign matter floating around the movement.
After all that is properly accounted for, there are still going to be infinitesimal differences in the build of each movement. These are due both to infinitesimal differences in the individual components used to build each movement, and minute differences in alignments - even with locating pins and shoulders.
These small differences are enough to cause differences in how each movement runs before adjusting. Therefore, a watchmaker is required to make those final adjustments and bring the replica movement into spec so it can pass the COSC tests. That's where - surprise! - "Adjusted in five positions" originated from on replica rolex movements.