I cant see the design of the iPhone 7 staying the same, every iPhone number (i.e. the 4,5,6 and so on) has had a different design. I cant see a reason why Apple would keep it the same as the 6 and 6S. I could be wrong but i don't think it is correct, i think Apple are probably still deciding before the final iPhone 7 design is settled and goes into production.
I can think of one. Possibly an oled screen for 2017 requiring a completely new case with Apple waiting on a complete redesign for it.