Why some RAM for the iMac 27 inch is 1.5V and some are 1.35V? I just got a new iMac 27 inch, and from 

http://support.apple.com/kb/ht1423#1
http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=746859E0A5CA7304

if I buy those 8GB, then I can add them to the original 4GB and make it 12GB, it seems.  

But I wonder why these are 1.5V and the crucial website also list these other RAM which is 1.35V for the same system:

http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=06C539BDA5CA7304

It just seems strange that either 1.5V RAM or 1.35V RAM are both ok.  Does somebody know why?

(it also seems that I can buy the 8GB set, and then the 4GB set http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=C330E0DDA5CA7304 and the computer will now have 16GB?  For an extra $26 I am tempted to do it, to have 16GB instead of 12GB, but it just seems weird to have that much RAM... can the computer use that much RAM?  Even if I run a virtual machine for Windows 7 for 4GB and one for Ubuntu for 2GB, that's only 6GB)
See my answer in this question on whether you can actually use the RAM and how to check how much RAM your computer is using at the moment.  It may be the case that you aren't even using what you currently have.  With regard to the RAM voltages, I'm not sure why they would be different apart from the sticks being slightly different designs.  You would be best off matching the sticks you buy to whatever voltage your current RAM is though.