System Profiler says the amount of space used on my hard drive is different than what Finder Get Info says. Why are they not the same?
If you have Time Machine Local Snapshots enabled (and I'm fairly sure it's enabled by default), then it's going to use some of your free space for creating these local snapshots. However, Finder doesn't take the space used by this feature into account, pretending it is actually free, because if the user starts filling up the disk, then the local snapshots get deleted -- it will only use `idle' free space, so to speak. See this other question for more details.
So, don't worry, if you actually need the extra space reported by Finder, then it will be made available to you when the need arises.