First, if Vaud were running Snow Leopard, that's not the case--it uses decimal, 1 billion byte Gigabytes. Second, a binary GB is actually closer to 1.074 billion bytes, because 1 KB=1024 bytes, 1MB=1024KB, and 1GB=1024MB. This is why the 120GB drive Vaud has is reporting 111GB available (meaning you're NOT running Snow Leopard, obviously!).
Now for the actual answer: Doing a Finder "get info" only reports file sizes for files you have permission to read. Low-level system stuff for which you do not have permission won't, I believe, be included in that total, nor will files owned by any other users on the computer.
So if there are other users on the computer, some of that space may be in their files. More likely, though, given that the discrepancy is only about 5GB, it's just system stuff. Totally normal, and that's a reasonable number (I don't know exactly what a full OS install uses, but it's much more than that).