This is a very complex situation with lots of factors.

-SFO being UA's hub and has less competition than say NYC-LON means UA can charge more. 3 airlines with about 4 flights a day vs 5 airlines flying multiple flights a day each.

-Tickets are sold by fare buckets. As the cheaper ones sell out the higher fares remain. Less seats on a route = less cheaper fares = faster sell out of those cheaper fares.

-SEA-SFO-HKG is cheaper has more of the lower fare bucket are available as they aren't in as high demand AND to attract attention away from competition as SEA isn't a UA Hub and has tech & aerospace companies.

You could probably get a cheap 1 way to SEA to start your flight off and then upon return to SFO ditch the remaining segment. As long as you don't do this often the airlines won't give you a hassle. Skipping the SEA-SFO will cancel the whole ticket.

Basically combine Supply & Demand crossed with competition.