Purebred dogs are selectively bred for certain physical characteristics and skill sets. Sighthounds haven't "needed" or been selectively bred for their receptive language skills like some other breeds. They can all learn but the learning curve is different IMHO. And, of course, within a breed you'll have some individuals that are quicker to learn new things than others.
I've now had 5 retired racers in my life and the "smartest" by far is Princess. She is incredibly social and oriented to words and cues - more so than any of the others I've lived with. She makes intense eye contact everyone and seems to actively listen to what you say to her. She's the only greyhound I've had that does this. Everyone that meets her, especially other greyhound owners, comments on it. Her eye contact reminds me somewhat of my beloved weimaraner, Hannah.
Hannah was often too smart for my own good! She learned a couple of new words or phrases every day even into her senior years. It didn't take her long to recognize familiar words when I spelled them and not only did she know her name (no big feat I know), but she knew the other dogs' names! When I would say their names, she would look at them and then back at me and then back at them! I was talking on the phone to a friend one day about my new next door neighbor and her two dogs & said their names. She jumped up and ran to the window and looked out at their house wagging her tail furiously hoping to see her friends. I was amazed that she knew what I was talking about! Combined with this intellect was a tremendous amount of energy and stamina that needed channeling constantly or she would find something to get into in order to entertain herself. I adored her beyond words, but in my older age, sighthounds are more my speed.