It comes down to different methods of sighting. Sayyid Sistani says that the moon has to be sighted with the naked eye; Sayyid Khamenei rules that using a telescope is permissible. Astronomists also say that technically the new moon starts before it can be seen by either of these, but generally the ruling is that it has to be sighted.
Another issue is the 'sharing of the same night'. This method used to be used. I.e. if a region shares a night, then the declaring of the moon from one part of that region would apply to all. This is now no longer being used as a criterion. Instead, Sayyid Sistani and Sayyid Khamenei say that you must follow your local horizon, so usually the moon can be seen in Southern Europe before it can in Northern Europe; hence, Spain will have Eid one day earlier than England.