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.
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.
With optical media like CDS, regardless of content or application, not all discs are of equal quality.

A CD reader will attempt to read at the fastest rate it can reliably without errors. How fast that is will vary widely not just per CD but even reading different parts of a single CD. Algorithms are used to determine how fast to read with low chance of error at any particular time. Different software products might each use different algorithms to determine that depending on the task at hand and how clever teh programmers are.
Rock fwiw with my gateway laptop running dbpoweramp the fastest rip speed I see indicated during ripping is 20x. Most discs seem to start ripping less than 10x fairly often then tend to get faster on later tracks. Not sure why that is but seems to be the pattern.