The Luci percentages all compute out to a percentage of 4MB, which would be around the 3.6M mounted as /overlay, left over from from the 16M flash memory not used by the firmware. I think the GL GUI might be reporting it as a percent of the entire flash memory.
If I do df -h, I have 10.5M used and 0 available in /dev/root, mounted as /rom, and 1.5M available of 3.8M in /dev/mtdblock6, mounted as /overlay.
Not sure how you have 6.3M in overlay, which implies more than 16M flash.