(1) If the assessments in a special assessment roll are insufficient for any reason, including the noncollection of the assessments, to pay for the improvement for which they were made or to pay the principal and interest on the bonds issued in anticipation of the collection of the special assessments, then the authority board shall make additional pro rata assessments to supply the deficiency, but the total amount assessed against any parcel of land shall not exceed the value of the benefits received from the improvement.

  (2) If the total amount collected on assessments is larger than necessary by more than 5% of the original roll, the surplus shall be prorated among the properties assessed in accordance with the amount assessed against each and refunded to the persons who are the respective record owners of the properties on the date of the passage of the resolution ordering the refund. A surplus of 5% or less may be retained by the authority and used for authority purposes or may be prorated and refunded as provided in this section.