Seems like you have not burned many dvd before just like hdd you will never get full/labelled capacity in a dvd because of the advertised capacity in normal number system but usable capacity in binary number system(aka 1000 vs 1024 factor).
how?
8.5gb=8500000000bytes(according to manufacturer's normal number system just like hdd or pen drive)=[8500000000/(1024*1024*1024)]=7.92gb.Unless you are using some kind of overburn option(available in almost all good & known dvd burning softwares) or your software is showing some rounded values it is not possible.Same is true for 4.7gb dvd(=4700000000bytes=4.38gb).