Why the Supreme's leftward drift?

Fred Barnes has an interesting article about Pres. Bush's efforts to make sure that Roberts would not, like Souter, turn out to be an extreme liberal. The article describes the interview process and the White House's concerns. What struck me, however, was this sentence:

He [Pres. Bush] decided Roberts would not lurch to the left as Souter had or even drift in that direction as other Supreme Court appointees of Republican presidents have.

A phrase such as the one I highlighted has to leave you asking why Republican nominees drift left. I can't remember hearing about a Democratic nominee who drifted right. I mean, think about it -- Earl Warren was an Eisenhower appointee, and Warren Burger was a Nixon appointee, yet both presided over the most activist Supreme Courts in American history. What is it about the air or culture of the Supreme Court that encourages this drift? Is it collegiality? In the rarified world of the Supreme Court, separated from the Sturm-und-Drang of politics and ordinary life, are they really have insights that are denied the rest of us? Certainly, it's something that we ought to think about, because I don't think it's a coincidence, and there may be useful answers hiding in those questions.