Why did the number of head injuries increase after helmets were introduced at the beginning of World War 1?

Field hospitals kept records of casualties. If a man arrived at the hospital with a head wound and lived, he would be listed as 'head wound', but if a man was dead, he would be listed as 'dead'. After the helmets came into service, many men were hit on the had and lived, and were listed as 'head wound', and so the number of the increase was made up of men who would have died and been listed as 'dead' had they not been wearing helmets. Helmets therefore decreased the number of dead, but increased the number listed as 'head wound'.