if you read carefully they are max rated speeds. and since a cd is spinning in solid circular motion, the outer parts will be read faster then inner rings. So the max performance is given at the most outer part of the CD. Unlike harddrives, the cd reads from inner ring to outer ring, and most read parts are on the inner ring. So you performance wont be at the rated Max performance all the time.
And this is same with all DVD and CD roms unless they are stated to be true xsomething speeds like kenwood cd drives. So, its not because some drives are cheap which gives them bad quality.