Why isn't prestige taken into account?

[Corbett]

Note, however, that the adjustments were based on LSAT scores, rather than the grades eventually earned in law school.

They apparently took grade inflation (or lack thereof) into consideration as well:

The formula assigns each college a "rank number’ based on how its students perform on the standardized law board exam, the LSAT, and how common a certain grade point average, say a 3.6 GPA, is at that school.