The left page is justified by default: the systems are spaced farther apart to fill the page. The right page is not stretched because it’s not full enough to trigger the threshold in Layout Options > Vertical Justification > “… when frame is at least [n]% full”.
Is all of the music in? If not, just keep entering it and the layout will improve. Or if that’s the length of it, you’ll want to make some choices about how best to lay it out.
Ben posted his very nice vertical justification guide at the bottom of this post: