A lot of those who come and leave are able to charge at the next SuperCharger on their journey, such as Manteca or a destination charger (displacing a low range Leaf, for instance). Some of the people who come and leave aren't as inconvenienced as those who would be forced to stay. Another possibility is that they need the charge at least one of the two ways in a two way journey, but it doesn't matter which direction they get it, so they check both ways and if the line is short enough they use it that time. For those two groups, as you point out, who knows how many users there would really be.