this isn't necessarily true. after all, peer/judge rankings are the most significant portion of the USNews ranking, and those rankings are affected by the graduates of the school getting out into the legal community and making a name for themselves and their school with the quality of their work. presumably, the top of the class from each school would be in the best position to make a positive impact on their peers with clerkships, biglaw appointments, and other top legal jobs. therefore, it could be rational to believe that keeping the top of the class has as big (if not more than) an impact as the entering class' LSAT/GPA.
the nuance to this however, is that most schools probably keep the top of their classes even without providing extra scholarship monies because that is where many of the top students will have the best opportunity to excel in their career. (apathy or ignorance about the transfer process could also be a reason many of the top students stay at their respective school).
so, for whatever the reason, some schools have not found it necessary or fruitful to provide extra money to top of the class students. knowing this, if financial considerations are paramount to a student, s/he should go to a school where they get a lot of money.