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Since the billing for all maintenance contracts is done at one time through the master contract, you should not have amounts displayed in both the Amount Billed and Amount Unbilled columns for a certain billing period. In other words, all maintenance contracts should either be billed or unbilled for each billing period.
An exception to this would be if a maintenance contract were on hold, while the master contract it was assigned to was billed. You would also see amounts in both columns if a new maintenance contract was assigned to an existing master contract after one or more billing periods had already been completed. Since the start and expiration dates must be the same for all maintenance contracts, you would see a figure in the Amount Unbilled column for the contract that was added after the billing was completed. The billing schedule for the newly added maintenance contract would need to be manually adjusted to remove the figure displayed in the Amount Unbilled column, unless the billing needed to be retroactive.