I am guessing that as the official launch date is not until November, they may not want the cars delivered to far in advance of that date. It may upet some people who have been told by dealers they will get their cars at launch if they see an M6 on the road 6 weeks early! I would guess they will do a rolling launch, but not as much as 6 weeks prior to launch! My dealer had their first M5 in for a good 10 days before BMW said they could let it go.
Thats my best guess anyway!