This seems to be a harmless case of erroneous old information slipping in because it was not stricken from all maps many years ago. However, it is not unusual for many maps to all contain the same mistake. Cartographers have been known to include small mistakes in their maps to catch people who copy their work. In the past, even if another cartographer found information on a map to be inaccurate, they would not always correct it completely, for similar reasons to why the value of the charge of an electron in Millikan oil drop experiment took years to correct.