An expense appears every month, so you label it fixed. Then the next bill arrives with a different amount. The apparent contradiction disappears when you separate two questions: how often it occurs and how its amount behaves.
Give frequency and amount separate columns
The CFPB’s teaching guide on fixed and variable expenses distinguishes generally stable amounts from expenses that vary. A recurring payment schedule does not, by itself, establish a constant amount.
In a fictional exercise, a service bills monthly according to usage. Write “monthly” under frequency and “varies with usage” under amount. That description is clearer than forcing the entire bill into one unexplained label.
Keep an estimate separate from a promise
Suppose three invented bills are 30, 45 and 36 units. Their average is 37, which can be a useful summary of those three observations. It does not guarantee that the next bill will be 37.
If you use an estimate in a learning budget, keep the observed values and the estimation method visible. The example illustrates recordkeeping, not a recommendation for how much any household should spend or set aside.
Read the agreement when the pattern changes
A change may require checking the billing period, usage, or terms rather than immediately assuming a budgeting error. Our article on matching a recurring charge to its agreement provides a useful next step.
Finish with two plain sentences: when the expense is expected, and how its amount was estimated. Those sentences make a worked example easier to understand and help prevent a regular date from being mistaken for a stable price.