Your WC premium is based on payroll. The premiums you have paid are based on estimated payroll. The audit is to determine what your actual payroll was for the policy period. If your payroll was higher than the estimated payroll, you will owe an additional premium. If it was less, they will owe you money.
General liability is the same way, only it is generally based on revenue. You really don't have a choice.
Some companies do a mail audit, where you send them the information they want instead of coming out.