What are "normal buffers"? A buffer design that sizes transistors for equal current strength has fairly symmetrical r/f time, you can check with commercially available logic ICs.
clock buffers having drive strength is more compare to normal buffers.and it's having doping concentration is also high.......
so high drive strength buffers having equal rise and fall times.....tr and tf
I have seen normal buffer also having same rise/fall transition this we call balance normal cells. But CTS cells are still special since its propagating delay are smaller than normal buffer of same drive and higher driving capability.