I get what you're saying, but the average is probably used because there isn't public information to do this in real time. Plus, the timescale of changes would

I get what you're saying, but the average is probably used because there isn't public information to do this in real time. Plus, the timescale of changes wouldn't work with this format and the author's intended message. You'd have things fairly constant for a few hours in a row, then +40 million one day and -60 million the next.