that is very normal, all drives cannot sustain a 16x burn all the way through
most drives however can sustain a solid 6x burn
6x and 8x is the fastest ill ever write dvds just because it stays a very steady burn all the way through
sorry tiny bit to drink
look up CLV writing on google and that should answer your question a lot better then i can
here is an quick explantion:
Zonal burning. It's pretty much impossible, at least with current hardware, to spin a disc at 16x from start to finish. Blame physics. With 4x and slower media, the drive spun up to the burn speed, and burned 4x from beginning to end, using the CLV (constant linear velocity) burning method. The disc also had a uniform look on the burned dye.
With the advent of 8x media, Z-CLV (zonal constant linear velocity) and P-CAV (partial constant angular velocity) were introduced. Z-CLV starts at a speed like 4x, then at a certain point in the media, jumps to 6x, then 8x, etc., until it reaches the maximum speed. Sometimes a 16x Z-CLV burn will only burn a few hundred MB at 16x, which is why "x" speeds mean almost nothing anymore. P-CAV is similar, but does not jump speed. It increases velocity from 4x to 4.5x to 5x, etc., until it reaches it's top speed. Much like Z-CLV, it may not hit max speed until the last 30 seconds worth of burning. This is why a 12x burn is almost an identical wait time to the speed of a 16x burn.
taken from this site here: http://www.digitalfaq.com/media/advancedconcepts.htm
its a 1/4 down the page