Because the opinions are circulated among the justices as they are prepared and, reportedly, sometimes revised to attract a convergence of opinion. The decision is not a "vote" - there is an initial vote but positions can (and sometimes do) change as opinions are circulated and debated.
Obviously Scalia's dissent had been revised between June 15 and its release, as it referenced the Obama non-deport order.
FU GEORGE W BUSH
FU OBAMA
FU ROBERTS