How does an audit work?

​When the policy is issued or renewed the premium is based on estimates provided before the policy begins. At the end of the policy term, the insurance company will review the policy and either charge or credit the policyholder based upon an audit of estimated figures.

​Examples of estimated auditable items include sales and payroll. Audits can be performed onsite by an auditor or via mail or telephone. A premium is charged for audit estimations.

At the end of the policy period if your estimates were too high then you were charged too much premium and therefore will be refunded, and same applies if you estimated too low then you would be charged more. 

And it's as simple as that!