If they know how much I owe, why don’t they send me a bill?
The last time I read about this, it was said that they don't actually know. They do statistical analysis and audit you if you are an outlier and didn't pay what people in your similar position payed. I don't know which metrics they look at though. Theoretically, this means if everyone decided to pay a fixed sum less, the data would work out and they wouldn't know who didn't pay enough. Don't take my word for it though.