From this guy (via Insty):
“There is Chief Justice Roberts, the institutionalist, who strategically guides the Supreme Court by not deciding more than is necessary to decide. Then there is Chief Justice Roberts, the institutionalist, who strategically guides the Supreme Court by deciding more than is necessary to decide. Which Roberts do we get in each case?”
Going by past decisions, we get the Roberts which is most likely to piss off conservatives in that particular case.