I haven't built that kit specifically, but others I have built had a similar instruction. It was because there was to be strips of thin balsa glued later, to the top and bottom of the ribs that butt up against the trailing edge piece. Does your wing have a similar feature? If so, you want to center the trailing edge on the ribs so the balsa sheet can be attached on both the top and bottom of the wing, by leaving an even amount of the T.E. sticking above and below the rib ends.
I hope that makes sense. []