No one is saying that's the purpose. Nonetheless, Hardware est is showing a clear discrepancy, and I think the OP is looking for an answer to it. Did he get lucky? Is there a hardware problem? Does Apple ship all of their systems with more VRAM and just disables what wasn't paid for?
There's a blurb about this on the Apple support site somewhere, but I'm too lazy to find it. Basically, you have 128, but the hardware test will sometimes get the number confused. Don't worry, you have less than it tells you 😀
Apple Hardware Test:
May incorrectly report more VRAM than actually present
Issue or symptom
Apple Hardware Test (AHT) may incorrectly report that the computer has more VRAM than is actually present. For example, AHT may report 256 MB VRAM for a computer that has only 128 MB VRAM. You may use Apple System Profiler to obtain accurate VRAM configuration.
Products affected
MacBook Pro (15-inch 2.4/2.2GHz) bundled with AHT version 3A121
Solution
To verify the accurate VRAM configuration:
Restart the computer from the hard disk.
Open Apple System Profiler from /Applications/Utilities/.
In Hardware and Graphics/Displays, see VRAM (Total) for the correct configuration.
Well. Nothing with the free additional 128 MB of VideoRAM...
As an interesting side note the hardware tests for a MacBook will also show 256 MB VRAM (which is the maximum possible amount of shared ram that the GMA950 will use). Although the System Profiler will show 64 MB which is probably some kind of reserved minimum amount.