Metric Gigabytes?

Here's what I don't get. I have a disk image that's 4.5G (it's a master, not a compressed image). I put in a blank DVD that'll hold 4.7G. Disk Utility says it doesn't have enough free space. Is somebody using metric gigabytes and the other Imperial gigabytes? Sometimes I wish Apple would be a little more explanative explanatory (spellchecker didn't undersquiggle it, so I thought it might be a word). Don't assume the user is a grandma—at least offer a reason like "the DVD requires n bytes to hold the volume header" or something that tells me why 4.5G won't fit into a 4.7G hole.

So I had to waste a dual–layer DVD just so it'd fit. That's dumb.

Broont, I'll go mail this during lunch.