Yes, the 5S production can't keep up with demand. They had to switch some production over from 5C to 5S. They still haven't filled sales from the first day yet and have to build inventory for the rest of the countries that will sell the 5C and 5S later this month. There may be a shortage later of the 5C because of the huge volume of sales of both phones.
They have the problem of not being able to produce enough phones to meet demand. That's it period. They have an ample supply of 5C at this moment but not the 5S. If the 5S is selling 4 or 5 times that of the 5C like some have stated, the number of sales for both is huge.
I would not be at all surprised if 5c demand picks up for the holidays. This is going to be the replacement for every kid who would have asked for an iPod touch for Christmas. I know all my nieces, nephews and friends kids who are tweens - teens have been saying this is all they want this year.