I think you're right. It makes sense that clerkships would pull a school's "career prospects" rating down if it was based on aggregate numbers in the private sector and their median salaries. UChicago has a far higher clerkship rate than NW, but that probably means NW grads get hired in the Chicago market at a similar rate as UChicago grads. Similarly, Mich seems like it would completely dominate its local market, and their OCI sounds ridiculously good, but they have less clerks and academics than Harvard.
An accurate top-ten for career prospects depends on your personal career goals, but in any true sense it's probably the T10 USNWR schools. If those Harvard grads didn't want the clerkships they could probably take most anyone's firm job. Same thing for Yale and Stanford. Clearly if you want to teach (or be a SCOTUS judge, for that matter) Northwestern is not the best school to attend!