Volume information. If you use, I believe, Disk Utility or Toast I believe you can get more (4.4 with Toast). Same way my 60GB harddrive is only 55.6GB in capacity.
DVD-5
DVD-5 is a single sided single layer DVD that stores up to about 4 707 000 000 bytes and that is 4.38 GigaBytes where 1 kilobyte is 1024 bytes(4 707 000 000B / 1024 = about 4 596 679KB / 1024 = about 4489MB / 1024 = about 4.38GB) . Video DVD, DVD-R/W and DVD+R/W supports this format. Often referred to as "single sided, single layer".