He's doing 6A->5B because he's unsure if a 6C will hit. Depending on what kind of angle you hit them with 6A, 6C will sometime's whiff. 5B is the safer hit confirm, though you do lose knockdown and 1k more damage.
However, at that angle/height R-1 6A'ed the Haku, it would have easily combo'ed into 6C.