The use of an alias was not uncommon. The reasons for using an alias could be anything ranging from 'running away from family responsibilities' to a soldier that deserted then decided to re-enlist in a different regiment without accepting the punishment for his desertion.
Re his story on being awarded a DCM -- it wouldn't be the first time that a man exaggerated his war experience and awarded a gallantry medal to himself. (thankfully not many!) This could explain his story about pawning the medals.
After reading more I see that, but my puzzlement remains. Being eighty years old, I knew the man, a wreck of a person after gassing in WWI, or so it was said, his wife dying 15 years prior to his demise, potentially from looking after him.
Jack was the only living son of Irish immigrants when WWI began, and previously I had found his attestations to join the Army, but nothing other, so gave up. When suddenly finding a "Rollie" Charrington with matching personality, my interest was rekindled, but now I can't find those attestations to perhaps discover why he joined with an alias.
My father's side of the family was larger with virtually all seemingly involved one way or another in wars, and sadly several were lost. My father was in Atlantic Convoys with MN and survived, his elder brother in Arctic Convoy with RN and lost. Most said little or nothing about what happened and silence from those lost.