Why do some memorials have the dates 1914-1919 or 1921, rather than 1914-1918 inscribed on them?

Many communities considered the First World War to be over when the Armistice was agreed (11 November 1918), when the Peace Treaty of Versailles was signed (28 June 1919) or when Parliament officially declared the war ended (31 August 1921). Some British troops continued to serve during the 1920s in the Army of Occupation in Germany, in Palestine or in North Russia so some war memorials may use other dates that are relevant to them.