Not every cd has the same amount of data on it. A cd with 30 minutes of music on it will take less time to rip than a 60 minute cd. Also, you get better rips at slower speeds. dBpoweramp may be set to burn at a speed that's less than what the drive's top speed is.
Ripping speed variation
Why does the ripping speed vary so much from one cd to another? I rip CDs using dBpoweramp on my HP desktop computer. And it seems the actual ripping speed is slower than the capabilities of the drive (which I recently replaced because it was slow). Replacing a 7 year old drive with new one did not improve speed.