Ina Fried reporting for All Things D:

A source at a second carrier also complained of extremely disappointing shipments from Apple, but said that demand for the high-end iPhone may also be slightly less than initially thought, too. Apparently it is the colors of the iPhone 5c that seem to have captured more customer’s eye.

I saw Apple’s decision to allow preorders on the iPhone 5c but not the 5s as a sign that the 5c will be in much greater demand than the 5s is. Poor availability could also be to blame. But Ina Fried’s sources come from the carriers, not Apple, and I think Apple will have the lion’s share of the inventory on launch day.