According to the specifications page, this router has 256mb of flash storage. However I was checking on the free space available on mine, and it seems like its boot volume only has 70mb.
Is there something wrong with my unit?
nope.
read the Flash MB paragraph on top of https://openwrt.org/toh/start.
you can also do a cat /proc/mtd via ssh, to see where the storage's gone.
70MB isn't "very little space".