I think they rate disk in terms of so called rotation speed - but what's really important is the velocity/rate the dye used is rated at. I also use Imgburn - and if you also have DVDInfo you view the velocity charts of a burn, they aren't linear.
I concur about using Imgburn. I've found that the little test it does in evaluating an individual disks burn speed capacity is quite accurate and can really speed things up. And, if it rates a disk slower than the diskmakers claim, I don't buy those disks anymore, or I buy them because they're a good deal but they're not going to deliver that speed.