Apple commonly employs a "Good Better Best" strategy when it decides what SKUs to offer for new products.
It reflects the fact that three different versions of the same product covers the market much better than a single version. The Good model is the entry-price one, the Best model is a fully spec'd out SKU for pro or well-off customers, and the Better model is a happy medium.
With Macs, it wasn't uncommon to see major new features only available on the more expensive models and unavailable on the entry-price one. When the iMac was refreshed in 2012, you could not get a Fusion Drive in the cheapest model.
Having different features at the different price levels is rare with the iOS products. It's so much cleaner to just differentiate Good, Better and Best by offering three different storage capacities.
The cheapest iOS product, the iPod touch, has seen this happen though. As Phil Schiller points out in this keynote, $199 is a magic price point, passing a psychological barrier for many consumers.
The first two generations of iPod touch had entry prices of $299 and $229 respectively. It would have likely been difficult to maintain margins by lowering the third generation to $199 and still update it to include the iPhone 3GS's internals. They opted to only offer those features, like a faster processor and more RAM on the Better and Best models, where prices started at $299.
For the Good entry-level model, they kept the second-generation iPod touch around for another year and lowered the price to $199.
History repeated itself a few years later. When the fourth-generation iPod touch added a retina display and cameras, Apple knew the third-generation wouldn't sell at pretty much any price. They updated the entire Good, Better, Best lineup to be identical other than capacity, and reset the entry price back up to $229 again. Then when the fifth-generation iPod touch came out in 2012 with a larger 4" screen, they once again bisected the lineup, keeping the 3.5" fourth-generation model as their Good one for $199.
Only with the sixth generation iPod touch, released without a keynote in 2015, did Apple finally debut a consistent new line-up of iPod touches, starting at $199.