I'm not sure this is always the case, though it does tend to be true. What happens is that the custodial parent gets a fixed sum each month, based on a formula. The non-custodial parent need pay no more. However, the custodial parent must care for the children, and this always entails spending far more than is provided by the non-custodial parent. The custodial parent has no choice but to provide for the needs of the children, no matter how much more is involved. In addition, the custodial parent must spend time with the children, giving up potential income, or often must pay for childcare in order to work. This tends to create a situation in which the custodial parent has a difficult time making ends meet.