I know that they have 1 OS to maintain, but isn't the hardware different from the iphone 3 to the iphone 4 to the iphone 4s to the iphone 5 to the iphone 5s and iphone 5c to the iphone 6 and iphone 6 plus? wouldn't those require similar firmware updates?
And the less capable models don't get all of the improvements from the OS and/or firmware. Same thing is happening with WP. MS just came out and finally cleared it up by saying Denim is the same as Cyan on the Models that don't have the power to use the new camera features. This also means if you already have Cyan just install Preview for Developers and you are at "Denim" on all devices except the 1520, Icon and 930. This could be why VZW was semi quick to release Denim in the US (except for the Icon) because they never released Cyan. AT&T has very little to gain by releasing Denim on most models (all except the 1520) because the users can already get 100% of the features by simply installing PfD.