So, back in 2003 I built my first system with 1GB of RAM. Initially I thought I would save money by re-using a number of existing components, including the power supply.
Sadly, the existing power supply proved inadequate and the system would crash shortly after booting, if it even booted at all. The memory was a single DIMM so there wasn't anything I could remove to reduce power consumption, however it turned out that telling the kernel to use only about 800M allowed the system to remain completely stable for the few days it took to get a new power supply.
Thus I conclude that the most likely explanation is that RAM which the OS doesn't believe even exists actually does use less power than RAM which is simply not in use at the time.