One possibility is size. According to College Navigator, UChicago enrolls 6,001 undergraduates, while NU is more than 46% larger, at 8,791 undergraduates. So just on that basis, you might expect to see larger numbers of med school applicants from NU.
Also, it wouldn’t surprise me if NU grads were generally more professionally-oriented and more likely to apply to MD programs, and if UChicago grads were generally more research-oriented and more likely to target PhD programs. If so, that would further increase the discrepancy in med school applicant numbers. But I don’t have any numbers to back up that suggestion.