I am looking to book a return ticket HKG-SDQ on UA website. When I search for HKG-SDQ depart Apr 2 return Apr 9 the following is the cheapest ticket with only 1 stopover.

Apr 2 HKG-EWR (S)
Apr 3 EWR-SDQ (K)
Apr 9 SDQ-EWR (L)
Apr 10 EWR-HKG (S)
Price is $1,949.62

It seems to be significantly cheaper if I break it up and book 2 separate return tickets HKG-EWR and EWR-SDQ. This is what I get.

Apr 2 HKG-EWR (K)
Apr 10 EWR-HKG (K)
Price is $873.86

Apr 3 EWR-SDQ (K)
Apr 9 SDQ-EWR (L)
Price $435.76

The price of buying 2 return tickets is $873.86 + $435.76 = $1,309.62. Quite a significant savings of $640 verses the single ticket for $1,949.62.

The flights are exactly the same UA180, UA1470, UA1473, UA179. The only difference is that I can book class K HKG-EWR if booking HKG-EWR but it's class S on that HKG-EWR segment if booking HKG-SDQ.

Is this a bug in the website or it supposed to be like this?