Problem burning data DVD's with Nero 5.5

I have the newest Nero out there and a DVR-A04. DVD 1x media, 4.7 GB. Yet Nero puts the cap at 4.5 GB of info. The little red line on the data “timeline” at the bottom is at 4.5 GB, not at 4.7. Why don’t I get the full 4.7 GB capacity?