Help with a law question please?

reconciliation of Gratz v. Bollinger (an affirmative action case involving U of Michigan undergrads) and Grutter v. Bollinger (an affirmative action case involving U of Michigan law school.) The undergrad AA system was struck down in Gratz. The law school's AA system was upheld in Grutter as constitutional (pes EP muster). How is this possible? Is this rational? Justified by the differences between the systems?