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Supply and demand. Apple was waiting on a supply of chips for the MBP and had to take a lot of orders that it hasn't been able to fill yet. On the other hand, it was able to ramp up ahead of time with the mini, and the demand hasn't had time to adversely affect the shipping times.
 
Demand for the new Mac Pro has been tremendous. Every time I stop at my local Apple Store people are hanging out asking where they are and getting on the waiting list
 
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