There are studies done that track the number of OEM machines with Windows sold and so on, but they are rare, that is why I haven't stated one. And even these aren't entirely accurate, but still I would trust them more than Net Applications statistics.
Did I mention "active" machines? One that is being used? How do you know when a computer is thrown out and a new one bought? How do you know if I have a PC sitting idle when I am using a Mac or vice versa? What about machines built by the consumer him/herself? OEM sales can also be misleading.
Yes, Net Applications is not really much of a source, but it's the best indicator out there.
I don't get why you think Upgrade versions aren't full featured. You can even do a clean install, they just require that you already own a copy of Vista, just like the Snow Leopard prices require a copy of Leopard. Oh wait I get it, you think that Ultimate is the only version worth getting? Please, a small minority need all the features, that is why MS offer different versions.
1. Mac OS X DVDs are not "upgrade" versions of the product. As long as you have a machine that can boot to, Mac OS X you can install from a clean slate. (as demonstrated by EFI emulation Hackint0shes. You can try EFI-X for an easy solution)
2. "Cripple" versions are precisely that. Versions with certain features disabled/missing. If you need those features, you'll have to "upgrade". Features such as UI Languages (MUI), Backup, Encrypted file system, etc; make the user buy upgrades if they need them—precisely the reason for the differences.
3. OS X has everything in the package. As you have noticed out there Windows is one of the lone OS's who does this "different editions" thing...
4. Besides, Windows' upgrade is the one whose cost needs to be justified—consumers have no previous good experience/good publicity to rely on to make the choice...
5. OS X's kernel is 64-bit rewritten, Grand Central, OpenCL, as another post mentioned, in addition to most of what 7 changed (incl. refinements/performance improvements), and they're offering a full version at $29; and Microsoft, with all their money, weight, and R&D dollars... can only come up with desktop changes, UI rebrand, and input support? Geez. Ain't worth the 49 (note: nearly double) for an "upgrade" disc if you ask me.
6. OS X isn't bad to begin with. Vista is known for the bad backwards compatibility, UAC, Memory & CPU usage, bloat, and overhype (removed Ultimate features?) as well as the whole "vista capable" scandal... sigh.