Yes, If you read ancestry's small print
About British Army WWI Service Records, 1914-1920
Approximately 5 million men served in the British Army in World War One (WWI). This database contains the surviving service records of non-commissioned officers and other ranks who served in WWI and did not re-enlist in the Army prior to World War II. With the final release, this database now contains the entire service records collection.
So they do not have officers service records,nor indeed any other soldier who re-enlisted (you can get the re-enlisted other ranks from MOD for £30, they all exist as they were stored elsewhere when WW1 records were destroyed by bombing many years later