Large discrepency between my real and available memory. Where do I start trouble shooting?

My server has 24GB of ECC-RAM in it. The sticks are physically there, the sticks passed Memtest with flying colors, but FreeNAS doesn't use all the memory (even though it sees it all).

In the FreeNAS GUI it indicates that there is 16GB of system RAM. When I run the following command:

grep memory /var/run/dmesg.boot

I get the following output:

real memory = 25773998080 (24580 MB)
avail memory = 16505761792 (15741 MB)

So the RAM passed Memtest (and was accounted for...

Large discrepency between my real and available memory. Where do I start trouble shooting?