Here's how Scotusblog's Lyle Denniston explains how such a result might be justified.

But the Texas plan goes considerably further than [Michigan did in] seeking diversity across the entering class of students; the plan also seeks to achieve that goal among the major fields of study, and at the classroom level. That extension of what might be called the "Grutter principle" of diversity is a key issue in the case taken to the Court by Abigail Noel Fisher, a Sugar Land, Texas, student who did not gain admission to the Texas campus and contended in her lawsuit that she was denied because she is white. Minority students with lower grade averages than hers got in under the plan, she has contended.

And here, instructively, is how a dissent at the 5th U.S. Circuit Court of Appeals last year framed the difference between the University of Texas' policy and the Michigan Law School policy. Criticizing the 5th Circuit ruling upon which Tuesday's action was based, a decision that upheld UT's admissions policy, 5th Circuit Chief Judge Edith H. Jones wrote:

The panel's opinion, however, extends Grutter in three ways. First, it adopts a new "serious good faith consideration" standard of review, watering down Grutter's reliance on strict narrow tailoring. Second, it authorizes the University's race-conscious admissions program although a race-neutral state law (the Top Ten Percent Law) had already fostered increased campus racial diversity. Finally, the panel appears to countenance an unachievable and unrealistic goal of racial diversity at the classroom level to support the University's race-conscious policy. This decision in effect gives a green light to all public higher education institutions in this circuit, and perhaps beyond, to administer racially conscious admissions programs without following the narrow tailoring that Grutter requires.

A limited ruling in Fisher, which strikes down UT's policy while keeping alive the rationale and formula endorsed by the majority in Grutter, would likely be seen as a victory of sorts for the tens of millions of Americans who are in favor of affirmative action in academia. But at the same time, such a ruling would likely be seen as a major disappointment to the tens of millions of Americans who believe that the time has come for higher education in America to do away with such policies in all their forms.

Why would the anti-"quota" crowd be disappointed with a ruling that strikes down UT's race-based admissions policy? Because there are signs here that point to the possibility of a more sweeping pronouncement from the court's conservatives, one that would gut both the Texas policy and the court's precedent in Grutter. It's all about the math. First, Justice O'Connor is gone from the court, replaced by the far more conservative Justice Samuel Alito, who already has expressed hostility to race-based policies.