NULL usage in water consumption data set


I’m wondering what everyone thinks might be the reason behind null values for the water consumption data set.
There are customers with 0 water usage, but what do you think the null values represent in this data set?

Is it that they used absolutely no water but still payed the 20 DH service charge (maybe the 0 cm h20 used means they used less than 1 cubic meter), or that they aren’t somehow ‘in the system’ yet?