It could easily be 1918 as some of the region was still under...

It could easily be 1918 as some of the region was still under Ottoman control than British. I assume they use the 1917 start date with how the region was divided ethnically under the Ottomans just before the transition period of ownership to the British and French colonial powers from WW1 onwards to 1948 to independence etc. The dates are key regional transition periods