why do dual layer dvds only hold 8.5GB?

[mcryptic]

i was looking at the press release pictures from philips' dual layer dvd burner and i found this pic showing a disk with large spaces in-between the tracks. Then i realized that they only hold 8.5GB, when a single layer dvd hold 4.7GB, so why can't they hold 9.4GB?

 

[ufot]

single layer holds 4.3gigs when you burn something to it. 4.3x2 = about 8.6

 

[mcryptic]

4.7GB dvds hold ~4480MB, i think dual layer dvds hold less because of the way the tracks are in the second layer. I wanted to know why they are like that.

 

[TechHead]

4480 MB != 4.7GB, but actually 4.38GB.

 

[mcryptic]

either way 4.38 * 2 != 8.5

surely someone knows why this is...