I don't think there's any need to worry. I see the same on my server, and I assume that FreeNAS is using as much RAM as it needs to.
Look at it as "I have 2GB of RAM available in case s*it happens".
I run an Intel C2Q 9400 w/ 8GB RAM and 6x 2TB Barracuda Green drives that I thrash pretty hard, but I don't see much variation in RAM used.