SetReward has same effect with AddReward

In my Project, I use SetReward and to set agent reward to 0 when there is a draw, but I found SetReward has the same effect with AddReward, Why?

public override void OnActionReceived(float[ ] vectorAction)
{
if (StepCount == 1000)
{
AddReward(1f);
print(GetCumulativeReward());
}
if (StepCount == 2000)
{
SetReward(0f);
print(GetCumulativeReward());
return;
}
if(!IsLive) return;
AddReward(-1f/MaxStep);

MoveAgent(vectorAction);
}

the result is 1 and 1, but I thought it should be 1 and 0

oh I have known the reason, the difference is only in same time-step