Plating carrier should construct the fare using its own fare but sometimes they just use the other carrier's fare which makes one's price lower if the fares differ, other than that you can aparently play with code-shares and presumably get different totals given a set itinerary. Other than playing with changing itinerary or code-shares (presumably) and operating carriers btw the points, I don't see what travel agents can add that AA RTW desk can't.