When you see that pattern, the most likely reason is favorable winds. Those flights are blocked to account for heavy winter headwinds. When they don’t occur, they can delay departure, keeping the arrival constant, or they can arrive early. They will often prefer to delay the departure, as they may have more flexibility to accommodate the extra aircraft at SFO than they do at TPE.

You can see the same pattern on most of the SFO-SIN flights recently, and those are on the 789, not the 77W.