because there is no 32GB iPhone 6s right now. that is the new 6s 32gb that will be out for preorder on friday the 9th. and released on the 16th along with the iPhone 7
When the 6s launched last year there was no 32GB model but there was a 128GB model. So for the 128GB model all they do is drop the price but they have to make the 32GB model so it'll be available along with the other new iPhones the 7 and 7 Plus.