The issue is that those numbers only tell a tiny portion of the story.. you cannot write an app for example to the 2.1 version of android and expect it to A) run on all 2.1 equipped devices or B) downgrade gracefully on 2.2 devices..
If you split out the 4 major percentage share holders to just the most basic differences you will rapidly discover that within 2.1 you have:
Standard with Google apps no custom UI devices
With Google apps and a custom UI devices
No google apps no custom UI devices
No Google apps custom UI devices..
Further you can break down the "with google apps + a custom UI to include at least the following:
Misc tiny marketshare customized versions..
And finally you have to take into account every single library change that was done by either the carrier for apps they have custom done to run on their version of a handset, as well as library changes done at the handset maker level..
Sure some of these devices will run apps coded to the 1.5 api flawlessly.. some however will definately not...
Sure you can segment within the android market based on whatever criteria you want as a developer.. but at that point you are not really creating "android" apps but rather you are creating Motorola Droid X or HTC Incredible apps.. which is a very different story than creating just an android app..
Keep in mind one of the "great benefits" of android was supposed to be to make life easier than "the way it was before" which was coding to specific API on specific carrier with various hurdles to jump through...
The general problem to sum this all up is.. you cannot have it both ways (and its just as true of windows and every other multiple device/multi manufacturer OS) "Android" being succesful can either be measured by how many devices it sells that are just as different from each other as the iphone is from a winphone7 or android phone (from a developer perspective) OR Google can attempt to reign in the versions and lock down what must exist to be called an android X.X device.. Else we are just comparing apples to oranges.. (just as having apple TV and iPad in the iOS numbers does not tell the real story, neither does lumping the G1 and the Samsung Behold in with the Droid and calling them all "android")