graduate admissions - Why do many PhD programs expect a higher average physics GRE score for international students than US students?

Browsing various PhD admissions programs, I consistently find that there is a higher expectation of international students on the physics GRE for admission. For example, at UT Austin, the average score on the physics GRE for students accepted for Fall 2011 was 907 for international students and 777 for US students.

Why is there such a difference in expectations?