Headline style used to be based on maximum impact for smallest physical space on the page. So it would be "Man walks on Moon" rather than "Man walks on the Moon". The internet meant essentially unlimited space so online newspapers returned to standard English for their headlines. Then Twitter came along with character limits and once again headline style was introduced to aid in maximum impact. The Daily Mail is big enough that they probably have someone re-writing headlines for Twitter, meaning that they can really go to town with their web-edition headlines.