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Not sure if it has been mentioned but perhaps the 5S was late to production hence why we saw so many 5C leaks
 
Don't be so sure. Remember that it took until like November last year before people were able to just walk into a store and purchase an iPhone 5. They were sold out everywhere at Apple stores and carrier stores for a while. Plus the long waiting times for those who ordered some online.

That was after the initial batch sold out.

Exactly, after initial batch sold out. Depending on your market, that could have taken 4+ days. My local Apple store had iPhone 5s for 5 days after release. After that, they were out for several weeks.

This exact same thing happen with the 4 and 4s....it's kind of the way Apple operates.
 
Probably the fact that they're supplying China on day one this time. That and/or the Touch ID sensor may be having production issues.
 
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