BMW uses on demand supply chain management. They do not stock any parts. All parts from all suppliers are ordered and delivered in exact quantities for current production, and even in the same order (I.e., if 2 cars on the assembly line will have black, then silver seats, then the 2 sets of seats come off the supply truck as black, then silver, in that order and quantity). This way they stock nothing, no need to store anything, etc. It is highly efficient but susceptible to this kind of shortage issue which would cause production delays and shuffling.
So it's possible the supplier has informed BMW that at a certain point in time, for a specific duration, they will not be able to deliver Carum Spice Grey leather seats and other interior pieces, and that timeframe coincides with 163 planned allocation build slots for the i8. So those allocations either have to use a different supply chain (Tera World) or be pushed back until the supply is available again. Rather than pushing all allocations, they are saying they will just move those affected backwards, produce less 2014 i8s and more 2015 i8s, depending on what people choose who are affected.
This is my rudimentary understanding
