The most selective colleges have a higher density of strong students and test takers (selected for in undergraduate admissions), so it is no surprise that they have lots of students who can earn high GPAs and LSAT scores.
Attendance at Yale undergraduate mostly says that the student was a strong student in high school, perhaps with some exceptional characteristic (which may have been unearned by the student, like relation to alumni, donor, VIP, etc.). Attendance at Yale law school mostly says that the student was a strong student as an undergraduate.