If one VIN gets delayed compared to others around it, chances are there were problems in production and it had to be repaired/fixed/replaced before going out the factory. It wasn't a precise science, and things did screw up along the way. For example, maybe one of the workers accidentally rams some equipment into the rear quarterpanel just before the stainless panels go on, damaging the underbody, engine compartment, cabin, glass, etc., and now it has to be placed on the sidelines until they decide what to replace and what to repair, and then take the time to actually do it. They specialized in constructing DeLoreans, not repairing them, so repairs could take quite a bit of time sometimes, especially with the more complicated repairs.