As some people here have mentioned, this is a carryover of the Seigneurial system. It has less to do with access to land than with shipping of commodities. All sites along the river (plantations and agricultural tracts) had waterfront access for loading onto flatboats. The parishes shown on this map are just upriver from New Orleans, where goods would be unloaded for sale. If you look at a current map of Ascension, St. James, St. John the Baptist, and St. Charles Parishes, the division of land has remained relatively unchanged up to now. The similarity with shotgun houses is just coincidence.