If you're voting Arrieta, you're basically saying it's because he has 20 wins (which Greinke could still get to). Greinke has a lower WHIP (0.85 to 0.90), lower ERA (1.65 to 1.88) and higher bWAR (8.7 to 7.4). Kershaw has a higher fWAR (7.8 to 6.2, though that's before tonight), strikes out way more 11.39 K/9 to 9.17), and has a better FIP (2.09 to 2.51). Both of the other two also walk fewer guys. I think the main reason Arrieta is getting such attention for the Cy Young despite Kershaw and Greinke's numbers being better is the timing of his dominant run. Best era in a 10 game stretch since Bob Gibson. That being said, it is an award for the entire year and it's not like the other 2 have been off their game during that time either.