It's mostly a semantics issue on what constitutes an end of a war. While the Armistice of Compiègne was signed on 11 November 1918, and ended the actual fighting of the war, the Treaty of Versailles was not signed until 28 June 1919. So, since the peace treaty was not officially negotiated until 1919, some people consider that to be the official end of the war, rather than going from the date of the end of combat, which had occurred the year earlier.

People can disagree on the official peace treaty date or the end of combat as the official end of a war, but often the peace treaty date is clearer and easier to mark. This is probably because the opposite of what happened in World War I is somewhat common, belligerents on either side may continue to engage in combat after the peace treaty. This can be either due to a lack of communication, as seen with the CSS Shenandoah, which continued to engage in combat with US ships months after the American Civil War was over, or because certain factions don't agree with the peace treaty and willingly continue hostilities, as exemplified by Anti Treaty IRA, who continued to fight for the 6 counties of Northern Ireland following the Irish War of Independence. Rather than give amorphous dates to the ends of those conflicts, most prefer the official dates of the peace treaties.

Follow-up question: I understand that British forces withdrew from the Russian Civil War, which overlapped with WW1, in 1919. Was this seen as a continuation of WW1 at the time? Is it possible that the British war memorials with 1919 on also commemorate the soldiers who died in Russia?