one of them is that not all companies assemble all their watches themselves, and partly rely on outside specialists (often to account for spikes in demand; the simpler pieces are assembled outside the brand to reserve capacities for the more demanding ones. QC is always at the manufactures). Several brands do this, although this is not actively communicated (e.g. Rado, TAG Heuer, but also Blancpain).
Rule: to really get a comprehensive picture, visit the suppliers are well - you might see surprising things...
Best,
Magnus