I looked in the sticky threads, but could not find an answer.

I'm trying to fly ord-sna-sfo-ord jun 16-17-20 and the flights i am looking at all show S9. If I try to book ord-sna-sfo all on june 16 and sfo-ord on june 20 then all segments show up S fare. However, once I try to make the sna-sfo part on june 17 instead of june 16, I get W and Q fares (more expensive).

Is there any way around this and why does it happen?
I guess ual.com sees i am doing circle instead of RT, but why will it not let me have the fare class?