Oberlin and Amherst undergrads go on to get phds in science and tech fields at about the same rate (7.5%). Carleton is right up there with Reed and Swarthmore at 12.5%. Of course, none of them come close to caltech (35%) and Harvey Mudd (24%)!
Northwestern is a large research institution, and many of it’s undergrads go on to get phds. But percentage-wise, it’s not in the top 50.
I just don’t think you can say that Amherst is weak in science.