Usually, the monthly cost remains roughly the same. If customer has credits that balance out the costs for specific items during those months. The final amount is shown, but if we organize the charges based on the specific types of expenses, we can see this more clearly. 

Here is the method to locate them in the Cost History console.

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.