I don't think that you're necessarily wrong to think that Berkeley and UCSB are "better" than Cornell in this field. However, i would note that there are dozens and dozens of subfields in theoretical astrophysics and each school has its own strengths. Cornell has a good program in general, but they also have several professors who are very highly respected in their fields (of course, the same is true for both Berkeley and UCSB). Rankings and reputation for astro departments are based on everything from observational facilities to amount of grant funding, so its not always easy to determine who has the best theory program based on these rankings. Oftentimes the best litmus test for assessing the Ph.D. program that is right for you is to determine what field you might be interested in and see what research is ongoing at in that field at each school (and in particular, what faculty is carrying out that research).
Overall I was more curious than anything.