Good question. When he was appointed, Roberts gave every appearance of being a good choice. And for a few years, he did indeed stand shoulder to shoulder with Alito, Scalia and Roberts.
Then, along came the 0bamacare case ...
And he switched sides faster and less explicably than did Benedict Arnold.
I think he's compromised, and responds to threats.
Well, perhaps you could consider the combination of Roberts (2/3) and Kennedy (1/3) as the third vote ...?