I think I'm not understanding what is missing for you. Perhaps it's the fact that the last map section, from Clarkston to Seaside, is numbered 7 and there are 8 maps in the set? Section 8 is not part of the main route. On its eastbound journey, the Corps took a different route back, section 8 represents this journey. It might be easier to see this by looking at the Overview and Detail maps available on the main Lewis & Clark Routes & Mapping page:
Hope this helps.