[Talks Too Much]

The SAT and ACT are tests for entry into American universities, the majority of applicants to which are American. The pool of international test-takers is much smaller and far more self-selecting than domestics. Honestly, I'm surprised the average international scores aren't much higher than they are.
In some US high schools, all students (even those not planning on attending college) are forced to take the test, and how motivated do you think they are to do well? They probably skew the numbers a bit.