In normal circumstances, most leases allow for advanced billing based on an estimate of charges for the year ahead.
This is to ensure there is appropriate funding in place to fulfil the management obligations. However, it usually is just an estimate. Whilst the lease will define more specifically how to handle any overspend or underspend, a balancing adjustment will be the difference between the initial bill issued based on an estimate of charges and the actual amount spent. This is otherwise referred to as a balancing charge.