It's not a quality control problem.
It's just that when you do FFF of the barrels against the
bolt-carrier and the rest of the GPMG, it ensures that evreything
is in place.
It's not one-size fits all.
Technicians need to do various steps in the FFF checklist to ensure
that both barrels can be certified to fire with the MG it is
assigned to.
There are also different types of nuts, no. of turns etc. Just as
your fingers are not of the same length, same goes for the
barrels.
A couple of mm make a whole difference, so that's where the barrel
nuts (of the barrel) come in. 
It's more complicated, definitely 
I
doubt I should answer further.
