Why Bosses are always right

I hate the fact that your boss is always right. I came into work last friday and I was speaking with my boss about possibly taking an extra shift on Monday, I told her that I could probably do it but then she never told me if I had the shift. I am a delivery guy and right after we discussed possibly filling in for the position on Monday, I went out on a delivery. When I came back to the store my boss was gone and left the shift on Monday still up in the air in my opinion. I hate the fact that if I wer to confront my boss about this I know I wouldnt be right, there would be no point. Just because I am scared of getting fired,I dont want to speak my mind with her. I know she didnt mean to screw me over, because now I have to go work, but she still will never realize that it was unfair what she did.