The enemies list is generated by looking for enemies who have around the same number of trophies as you, and in a similar Age. Earning trophies is a sign of your skill in battle, not how advanced your Settlement is, so as you gain more trophies you may find the opponents become more difficult as the game will try to match you with equally skilled players. If no enemies are found who are close to your trophies and Age, the searching can expand both the range of trophies and the Age of the opponents.