One needs to remember that the 'build week' is the 'scheduled build week' which is assigned (by Fisher production) shortly after they get the order from Chevrolet division. Some times it turns out exactly right, but other times 'things' interfere... ie. 'sold' orders take precedence over dealer orders, certain parts shortages, etc...
If you've ever special ordered a car, within a couple of days, the dealer can tell you an 'expected' time, based on the scheduled build week', but as the time draws near when it's being built-, they give you a more exact time.