Your friends are arranged in a leaderboard format based on their average percentage of reaching their goal for the previous 7 days. Because who doesn't like a little bit of friendly competition especially when it is helping you live healthier. This leaderboard changes often as people forget to drink water or as they hit a streak and meet their goal multiple days in a row.
It is not about drinking the most water, that's dangerous. It is about reaching or getting close to your goal consistently over time.