Nothing contained in the section shall be held to prohibit the Collector from deviating from a boundary as held by actual possession or as shown on a former map, and laying down a new boundary, if all the parties concerned agree to such new boundary, on the ground that the boundary held by actual possession, or as shown on the former map, was incorrect, and if it appears to the Collector that there is no objection to the adoption of such new boundary. The reason for every such deviation shall be recorded in the Collector's proceedings.