A Handfu of Dust

We have read 'A Handful of Dust' by Evelyn Waugh. My main question is why is a man who was born, Arthur Evelyn St. John Waugh, write under Evelyn. am I old fashion that I would think that he would go by Arthur or John?? Is it a generational difference? As far as the novel, this one was very good and I would recommend it over the other so far.