12MB is relatively nothing when you think of the total capacity of a DVD which is why i said ~ (approximately). also, NERO allows one to overburn a DVD all the way up to 4600MB (and more with a registry hack) on certain dvd burners that can overburn (e.g. Plextors).
as has been stated already....4,700,000,000 bytes is 4,488MB which is 4.37GB. NERO is showing you GB using 4488MB/1000 (instead of 1024) which is 4.488GB which is ~ 4.5GB...it's dependent on how the software displays sizes.
to answer your original question...you're not doing anything wrong and the DVDs you bought are just fine...
sorry to yell but IT'S ALL MARKETING SPEAK!!!!!