If you look more closely at those stats as presented in the original post, they aren't saying that there is a greater chance of a 4" snowstorm with AO+ as you go south (the likelihood is similar at all 3 locations), but rather it makes up a larger percentage of the 4" snowfalls as you go south. Put another way, it implies that NYC is more likely to see a 4" snowfall than either PHL or DCA when the AO is not positive.