It's not only about numbers, it's more like induced obsolescence. By putting less RAM, when iPad 3 will appear with 1GB RAM you will say "WoW! 1GB RAM, that is what I needed in the first place," you'll sell the iPad 2 and get the new iPad 3. You're iPad 2 will be an old machine, while iPad 1, without a dual/core processore will work much like my iPhone 3G on iOS 4, and that because of the induced obsolescence in the software. Another example is the CPU on Macbook Air's CPU - you are already buying a two years and a half old technology.
Apple was more about things that were still good over time, now in one year everything is obsolete, except Mac Pro, perhaps...