It is interesting because in most international rankings, which factor in research output quite heavily Princeton is in the top 10 usually. And yes while Princetin has a big undergraduate focus it does have many graduate programs that are top notch. maybe the fact that Princeton has no medical, law and business school could hurt it a bit in this ranking, but given the strength of the many graduate programs it does have I would still have expected it to rank top 30 if not top 20.