Compare the University of California and MIT. University of California has 250,000 students and MIT has 11,000. After normalizing the data, MIT produces 1 unicorn per 1,200 yearly enrolled student, while UoC produces 1 unicorn per 13,900 yearly enrolled student. By this list, MIT students are 10x more likely to found a unicorn compared to UoC students but are ranked lower in this list.
The authors shouldn't even be combining all ten UC schools in the first place as each runs as a separate university with their own faculty, admissions, etc.
A similar argument applies to the IITs.