The simple fact of the matter mostly likely is because that's how it works. Depending on your drive, its firmware version, and the type of media you're using, you can get different burn behavior. For instance, for anything above 4x, on several drives I've had, it was common to burn at 4x for 50% of the disc, then to up in 2x increments of a quarter of the remaining time, depending on the max. supported speed. I had like 2 years of this type of burning, so, I lay it down more on the drives.