Thanks for the comment! Yes you got the intuition behind it. I am using the prices that Apple sets for its one year old phones which they sell as brand new alongside their newest model. So right now the iPhone 6S, which was introduced September 2015, is sold new by Apple at a discount from the original price (since the introduction of the iPhone 7). The reason I had to adjust the iPhone 6S is that the entry level 6S is now the 32GB model, whereas in 2015 it was the 16GB. The base model 6S in 2015 was $650 for 16GB, while since September 2016 it is $550 for the 32GB, so the improvement from the extra storage space makes the 2016 base model better than the 2015 base odel. In my adjustment, I considered the storage increase “worth” an extra $50, so $150/$650 = 23%.
I averaged the price changes across the models. Generally, the more expensive the initial price of the model, the smaller the percentage change will be. So for example, the iPhone 6 Plus 64GB retailed at $850 when new in September 2014, and $750 when the 6S was introduced in September 2015. So this price change would only be $100/$850 = 12%. However, recently the more expensive versions actually got bigger price cuts: the 6S 128GB dropped from $850 in 2015 to $650 in 2016, and 6S plus 128GB dropped from $950 in 2015 to $750 in 2016. The percentages are $200/$850 = 24% for iPhone 6S 128GB, and $200/$950 = 21% for iPhone 6S 128GB. So those figures are part of the reason why the average is greater than 15% ($100/$650).