Interesting site that one! I found this on the subject:
Quick note: "4.7 GIG" vs 4.38GB disc size
Many people wonder why there 4.7 GIG discs are "defective" and "only" write 4.38 GB at maximum capacity. This is not an error of any kind, but rather an industry inconsistency. The 4.7 "GIG" that is calculated is actually 4.7 billion bytes. But on a computer, it takes 1024 bytes, not merely 1000, to make up a full MB.
The 4.7 GIG discs = 4.38 GB = 4489 MB (for DVD-R) and 4483 (for DVD+R)
This kind of inconsistency is found in many other areas. For example, the 120GB hard drive will format to about 112GB, requiring 8GB for file system and other settings. A 6-hour video tape is about 6:05 in length. And let's not forget the most famous one: hot dogs come in packs of 10 while buns come in packs of 8.
From
here.