My guess...
Orders are seperated into batches on pretty much a first come first served basis (or as much as is feasible regarding customer location and BTO options). The batches are sent to the factories to be fulfilled by several departments. Some departments have lower than expected yields causing delays for some first batch orders. Some departments have faster than expected yields, and they use the excess time to reshuffle some later orders if convenient by geographic destination. There are also a few cancelations which cause reshuffles - they'll have to find an order with matching BTO options to fulfil.
The reshuffles obviously cause a little variance in processing time for some people. There's no favouritism in play, only practicalities of mass production.