Not only that, but a significant amount of space on optical media is set aside for read error correction-- which is all to the good, but some of that space is counted by DVD manufacturers as data space.

(I'm not sure if the hard drive manufacturer practice of counting a gigabyte as 1000000000 bytes rather than 1073741824 bytes is followed by DVD folks, but it's possible, which would add up as well)

So... it might be a bit worse than that.