Not true. Its normally 9-10 days, just like this year. Last year was the only the second year that was a 2+ week gap.
iPhone 6 Event 9/9. Launch 9/19
iPhone 5s Event 9/10. Launch 9/20
iPhone 5 Event 9/12. Launch 9/21
iPhone 4s Event 10/4. Launch 10/14
Last time it was more than 10 days was iPhone 4, Event 6/7, launch 6/24.
OP - Even though its a similar body style, the entire body is still different. Every single time there is a body style change, supply is always shorter. the 's' models use the same frame so those already made and they just need to fill the internals. The new body requires new internals and a new body, increasing the time it takes to manufacture.
But also less variations, and less launch countries so less complexity (all info at launch of each model):
iPhone 4s: 1 size, 2 colours, 3 storage variations, 2 modem types = 12 variations for 7 countries.
iPhone 5: 1 size, 2 colours, 3 storage variations, 2 modem types = 12 variations for 9 countries.
iPhone 5S: 1 size, 2 colours, 3 storage variations, 4 modem types = 24 variations for 10 countries.
iPhone 6: 2 sizes, 2 colours, 3 storage variations, 3 modem types = 36 variations for 10 countries.
iPhone 6s: 2 sizes, 4 colours, 3 storage variations, 4 modem types = 96 variations for 12 countries.
iPhone 7: 2 sizes, 5 colours, 3 storage variations, 4 modem types = 120 variations for 25 countries.
So less different types of raw materials, less storage space per specific same variation raw materials (or more storage space for same variation raw materials), more capacity for manufacturing same variations, more storage space for completed same variations, more capacity for all forms of shipping of same variations, more capacity for all retail and carrier storage for same variations.....
9 - 10 days is still also considerably less than 14 days in the scheme of things - and then you have launch to pre-order and pre-order to release....