There are multiple leagues in the Arena, therefore the higher the league the greater the rewards! The leagues are as following:

Bronze league
Silver league
Gold league
Diamond league
Champion league
Legends league
Divine I
Divine II
Divine III

When a league week starts, all players are placed into leagues based on their final Trophy score at the end of the previous week. If your Trophy score is below the limit of your league at the end of the week, you will move down to a lower league. 

At the start of every Arena League week, your Trophy score is set to the limit for the division. This is why your score usually drops when you're promoted to a new league!