It wasn't assigned to a computer manufactured some time ago, it was assigned by a tech in some repair depot, and for all we know it happened a moment before.
The factory's serial assignment systems are connected to Apple's internal systems that would also have recorded the 'tech in some repair depot' assigning that number?
It doesn't literally have to be from the same factory, as it could be any one of the dozens of factories or hundreds of repair depots.
