One part of that is that while a particular brand/type of disk may be marked as "16x capable" that doesn't actually mean it will burn at that rate on all burners. Pioneer, for example, has a listing on their site (the firmware updates section if I remember right) that lists the media identifiers of types that will burn at different rates in their burners- and not always does the burn speed match what a type is marked as being "capable" of. Again on Pioneers they will only burn at near the top rate on a very small selection of disk types.
Also all disks will burn at different rates at different parts of the burn as the laser moves from the inside of the disk at the start of a burn to the outside at the end due, so the disk speed in Toast is averaged (as well as showing a momentary burn rate also) so it can be misleading.