Contests typically imply competition. And those who do best in a contest receive the "best" prize, which is the opposite of how the Lumnis "contest" works. Leaderboard would probably be more accurate.
Giving a reward to the top would result in more competition which would likely result in a higher average. Worst case, it gives those at the top more incentive to keep pushing which would likely raise the average some more, which everyone benefits from.And giving a reward to people who are already at the top would actually make the ratio worse.
It would also make the contest a contest instead of a leaderboard.