I noticed something strange

When I searched for SFO-SGN (on July 4), I saw FS/ CS- YS+ for CX 879 (SFO-HKG) -> CX 765 (HKG-SGN)

However, when I searched for SFO-HKG, I saw FS+ CS- YS- for CX 879.

What could cause this discrepancy? Does the economy award to SGN really exist?

Cathay website was able to bring up CX 879 & CX765 together for an economy award ticket, but I don't have Asia miles so will need to go through AA to book, and AA Explat desk only sees Z (first) inventory. Of course I can call back, but the discrepancy between 2 searches makes me less confident in what I am asking for.