Not quite. You are updating different versions of the same product - its still an upgrade. Its your only choice but the fact that version numbers changed (since Tiger and Leopard are two different products) is by definition an upgrade....
Look at windows. You can start with nothing and go to version 5.2 (Windows XP). That is a full version. You can also go from version 5.0 (2000) to 5.2. Its still the same Windows, but two separate products. You are basically arguing that you don't upgrade Windows since you already have Windows...
I'm completely confused on what you're trying to say here.
There are three versions of OSX out there:
1. The boxed version of Leopard. Or Leopard Server.
2. The 'upgrade' disk of Leopard you got if you purchased a Mac right around the changeover.
3. The disk that comes with your Mac as a "Restore."
Now, each of these contains OSX, but number 3 also contains your extras like Comic Life, iWork, and iLife.
In the Windows world, you can get:
1. The boxed version of Windows. With Vista, there are seven different versions of Vista.
2. The 'upgrade' disk.
3. The OEM disk.
4. The Restore disk.
In my experience, the OSX upgrade disk is a full version of OSX, but the Windows one is not. If you have a Leopard disk and want to install it on a Panther machine, you can. But, if you have a Windows XP Home disk you can have problems putting in on Windows 2000 Pro.
In the Mac world, if you have a a real disk, you can install OSX heedlessly on any machine you desire. In the Windows world, you can't unless you've managed to play the OS disk game just right. It's more complex.