Older iPhones outselling iPhone 5?

I've read various reports claiming that older iPhones (4S and 4 combined) are currently outselling the iPhone 5. While I don't doubt the claims I wonder what the reasoning is for consumers to choose the older model:

Let's say the 4S is the next best selling model. Off or on contract, there is a $100 dollar difference between the two 16GB models. Not really that much compared to the total price at the end of the contract. The economy isn't particularly worse than a year ago when tons of people sprung for the $200 iPhone 4S rather than the then $100 iPhone 4. So that $100 seems like not that big of a deal.

Could it be a screen size issue? Could those users actually prefer the smaller screen of the 4S? Or are the models too similar and maybe they don't think it's worth the extra hundred bucks? What do you all think?

[I'd provide links to articles but I have to search through my Twitter feed to find them. If you search for "old iPhone models outselling iPhone 5", the first thing that comes up is a BGR article and I don't feel like giving them page views.]

Edit: To add more stuff

Thank you to all who replied with your thoughts. Just wanted to add another observation: When the iPhone 4S was released the starting model had 16GB just like the current iPhone 5, but the iPhone 4 (next model down, $100 on contract) was 8GB as was the then free iPhone 3GS. Currently the $100 iPhone 4S has 16GB vs the $200 iPhone 5 with 16GB as well but the iPhone 4 is down to 8GB. Could this have been a major mistake on Apple's part? As far as most customers are concerned they're getting "the same" 16GB for $100 less. Perhaps they should have either knocked down the 4S to 8GB or at least had the iPhone 5 start at 32GB.