Today I occasionally figure out the reason, and here's why:
About"the manhattan CAT" problem, it is my fault, because although you can get a relatively high score of both verbal and math, your rank will not treak you. And I guess my classmate just reviewed all the questions about "the prep"(he got the whole question pool for preparation...and I saw it.), took them repeatedly and then get a 770.

I feel a little wierd to reply myself, but I did it, because I think some guys confused about the same problem may need it.