It is all about representation of size/space. Like how people complain that a 4TB drive will show up as 3-something TB when formatted. In those cases you’re not actually robbed of any space, just that the space was misrepresented by those marketing it (base 2 [computer ones and zeros] vs base 10 [people counting on their ten fingers]). To your computer a MegaByte is not 1,000 KiloBytes, but 1,024 KiloBytes (1GB is 1,024MB, 1TB is 1,024GB etc.), to a marketing firm, they pretend the transition between KB, MB, GB, TB, etc. happens at 1,000 increments rather than 1,024. Your OS has chosen to cater toward such advertised sizes, so as to make you think they’ve given you more when they really haven’t, and to appease people who are incapable of understanding why things don’t show up as their advertised sizes.

In, short, a DVD-DL advertises a base 10 size of 8.5GB, and that’s about how it will show up on your Mac. It isn’t exactly the best way of doing things, but that’s what’s going on.

(Also, I’ll clarify, 1,024 isn’t some weird, random number, it is a progression from: 2, 4, 8, 16, 32, 64, 128, 256, 512, 1,024. That’s why you see RAM and other types of memory/storage all the time with numbers like these, but the bigger things get, the more tempting it gets to claim something is bigger than it really is by rounding up with creative math, than to actually have to supply that extra storage.)