I believe some RAM is allocated to system files on boot so that the phone can start up. Also, the amount of physical space on a chip is different than the space a system can use. For instance any flash drive you get that is advertised as having 16 GB of space, actually has about ~14-15 GB, because they say that it has 1.6 billion bytes (or whatever the number is), but there are 1024 kb in 1 MB, and 1024 MB in 1 GB, plus the space needed for driver files lowers the total amount of space available to what you see. So all of that is probably what amounts to what you're seeing, I'm not an expert nor am I a developer, but I've looked into these things a bit.
Plus, you could just google search "why is my phone's ram less than it's supposed to be" and get a clearer answer!
Galaxy S III (Sprint)
ROM: M.O.A.R v9.0
Kernel: Optimized Stock
Recovery: TWRP 188.8.131.52