Why is it that there was always an S model of iPhones 4 to 6?

As far as I know iPhone 7 doesn't have an S model.

Apple adopted the model of releasing S models of phones every other year since the 2009 3GS. An improvement over the previous but keeping the same form factor.

There was the 3GS, 4s, 5s & 6s.

Assuming Apple keeps up with that there would be the 7s in 2017 but it is expected that they wouldn't & instead introduce an iPhone 8 (10th anniversary of the iPhone)