Lots of ways. Vegans are an extreme minority. I assume most medical diets are designed for the majority of people. They probably had to formulate a special diet for him that was vegan, and it may have taken a while to find the proper mix.
Also, statistical outlook data for cancers, surgeries, etc, are based on statistically average populations. Vegan people deviate that from that population, and that could mean their outlook/treatment be different.
Jobs' whipple surgery drastically changed his digestive system. And because it's at least sort of rareish, I am not sure any good data exists for proper procedure on vegans who have undergone a whipple surgery.