This is to encourage increased orders and promotions on the book from retailers. The second print will come out on September the 25th, the same day Saga #14 comes out. Along with the two trade paperbacks, that gives an added incentive for retailers to persuade customers to get on board.
While Image Comics has found success in its Dollar line of comics, reprinting the first issue of a series of a dollar, the increased discount means that Image will receive as much revenue on the $2.99 Saga #13 second print as they would on a $1 book, giving retailers the remaining profits.
Which should give retailers added incentive to push the book to enquiring minds… well it worked for issue 7.