It can look bad to have high test scores and poor grades because it implies you were a lazy student. It's not a sentiment that I agree with but our guidance counselors basically put it to us that way.
I applied mostly to Ivies and similarly competitive colleges. They get plenty of applicants with near-perfect test scores and GPAs, so my poor grades made me a subpar candidate.
This happened with people in the highly gifted magnet at my High School. It didn't matter that they were taking nothing but AP classes by teachers hell bent on being extremely difficult. They saw consistent C and B grades and a 1600/2400 (I went to High School during the transition from 2 part to 3 part SAT). Some didn't get accepted anywhere due to the fact they only applied two a few Ivy League schools.