It could possibly be that the SEA-MAN fare is a BD fare, not a UA fare, even though it involves UA on the SEA-ORD/ORD-SEA legs. BD fares should allow UA flights domestically, so you'd really have to see who the ticketing carrier is or who the fare is being published by, to see if it's a UA fare or BD fare. If it's a BD fare, then united.com would not show it.