It’s a fact tied in to exponential growth, during one doubling period, as much of whatever you’re tracking gets used as the entire history since that exponential growth started. That last bit is the key, human population is an exponential growth thing, but it hasn’t been uninterrupted or by a constant factor. There’s a long time when we were hunter/gatherers with a stable population and even in more modern ages, epidemics have reduced populations significantly.