I still think your view of fragmentation is too narrow.
Features in successive models do not always "stay the same or improve".
Making a screen taller or wider or adding and removing hardware buttons is not strictly backwards compatible improvements.
See for example the new Iphone screen. You can work around it with black borders, but if you want to have a native look and feel you need to do some work for each model.
Same with the Ipad screen. Even though it doubled in size so scaling was trivial, that just means it is backwards compatible with the old apps. If you want to create a new app that looks good on both versions you got to do extra work.
It's perfectly ordinary backward and forward compatibility work
By that logic all fragmentation is "perfectly ordinary sideways compatibility work".
To me Iphone 5 and Iphone 4s are products in parallel, since they are used at the same time.
Of course, after a while the old models become obscure and the user base so small that you can ignore them, but even though they were on sale in serial, they will be used in parallel for quite a while.
I guess it all depends on what you mean by "fragmentation". Two models sold in serial, but in use at the same time is the simplest case of fragmentation to me. 20 models sold in parallel is another much worse case of fragmentation.
But probably some people think "Two models is not fragmentation, because that is easy to handle. Its not fragmentation until it becomes a real burden". I just don't agree to that view. Just because it is worse on android does not mean it does not exist on the iProducts