have no answer as to why but my 3520 does same thing.
when using roxio disc copier it goes from 4.1 to 6 (at %40-45 level)
then around %65 it goes to 8.3.
when using DVD Builder it starts at 4, goes to 6 and towards last %30-40 of burn it fluctuates between7-8.
This is using TDK x8 DVD+R (moser baer media)
on my Maxell x8 DVD-R it starts at 8-9 and goes to 14-15 (when put on X8 setting)