well i believe its mainly for reliabilty reasons, in the case of the 24x, it will start at 16x (00:00) and then at i believe 8:00 it goes to 20x and then at 16:00 it maxes to 24x .. it does a verify check (media capability and data integrity) between each change of speed .. it only stays below 24x for the first 16minutes of the cd .., but ya i believe the main reason is the engineers decided that was the most reliable way, you can find more info at cdrlabs.com .. hope this helps ..
-neural