For example, as long as the Android source-code is open-source, there is no practical manner for Google to control, or prevent, its handset or carrier partners from grafting their own unique "skin" on top of whatever Android version is most current. More damaging, there is no means of ensuring that all Android handsets get timely upgrades. With the result that there are probably (at least) a half-dozen different releases, with different capabilities, extant at any one time.
The people this seems to matter most to are iPhone supporters, who are used to needing OS upgrades in order to have the latest capabilities... or not, depending on how much Apple wants us to buy the next model.
Android is a quite different situation from iOS, as Android has had all the primary capabilities (voice, video, multitasking, folders, etc) since almost version one. So there's not as much need for an upgrade for a phone. And even fragments for tablets is available as a library for older devices.
Android's upgrades are mostly about looks and speed, whereas iOS upgrades are often about adding basic functionality.
Moreover, Android phones always have the latest versions of important apps like Google Navigation, Maps, Search, because those things don't require an OS upgrade. Even integrating something like Facebook is easy to do on Android without needing an OS change. That's just the way Android was designed.
Finally, there are so many alternative themes and launchers for Android, that what a phone comes with (e.g. Sense or TouchWiz) isn't that important either... and the alternatives don't require jailbreaking (rooting) to install.
The upshot is, it's just not possible to directly compare Android upgrades with the so-called same-version iOS upgrades (which, even with the same version number, have not so common capabilities between iOS devices).