If the proceeds from any improvement district assessment levied for improvement purposes exceeds the final amount necessary for the purposes for which it was levied, the excess may be held in a special fund to be used for the operation of the works of the improvement district, be transferred to the general fund of the district to be used as the board deems proper for the benefit of the land in the improvement district, or be distributed to the holders of title to land in the improvement district in the proportion that the respective parcels of land contributed to the total collected from the improvement district assessment, the holders of title to be conclusively determined by the district assessment book last equalized at the time the distribution is made, with unknown and fictitiously named owners being disregarded.