I’m not sure about this but I think it might have to do with nominal time and nominal goal. When you set arrival position, it calculates the arrival points based on nominal goal, but when you use arrival time, it calculates the points based on nominal time.
By the way, using time points and arrival time points makes use of time points twice in the formula, which I think is not rational. Or am I wrong? What’s your motivation to use both time points and arrival time points?