Usually, the monthly cost remains roughly the same. There may be situations where credits balance out the costs for specific items during those months. The final amount is shown, but if data is organized based on the specific types of expenses it can illustrate charges in a easier to understand way.
If you want to determine whether the cost of a specific service is lower, select that service as a filter. Otherwise, if you want to check the overall cost, do not select any filters.
Categorize based on "Detailed Charge Type" to understand which charge type is causing the discrepancy. Usually, this discrepancy is related to how credits are applied: