First, it is certainly true that children with more economic resources, on average, end up better prepared for standardized tests. They tend to have better teachers, more resource-rich educational environments, more educated parents who can help them with school and, sometimes, expensive SAT tutoring.
Second, the test itself may be biased towards wealthier students. These tests tend to be written and evaluated by privileged individuals who may inadvertently include class-based knowledge, not just knowledge, in the exam (asking questions, for example, that rely on background information about golf instead of basketball).
In any case, this correlation should give us pause; it calls into question, quite profoundly, the extent to which the SAT is functioning as a fair measure. Perhaps it measures preparedness for college, but whether it measures potential is up for debate.