(a) When any special assessment roll proves insufficient to meet the costs of the improvement for which it was made, the City Council may make an additional pro rata assessment. The additional assessment shall not, in any case exceed thirty per cent (30%) of the original assessment.
(b) If the assessment proves larger than necessary by five per cent (5%) or less, the City Council may place the excess in the City Treasury. If more than five per cent (5%), the excess shall be refunded pro rata according to assessments.