Thanks Anurag. David D explained it in similar terms, and it makes sense to me now. I am glad I asked. To be fair, I am quite careful with water consumption anyway, and we have 2 massive rainwater butts. We have a water meter and only use and pay a fraction of what "they" estimate we should use. Every time we move house, they don't believe us and charge us 4x what we use. Then they have to pay me back. Thanks for taking the time to add to the conversation here. I hope other people read and "get" it.