The most probable reason is that your Highest Wave score was over 80. If so, please read on for the explanation.
There is a feature in place to automatically discard scores over 80 on both Google Play Games Service and Game Center. We do this because some people hack their games to pretend they've earned really high scores when in fact they haven't, and we don't want the leaderboards littered with these fake scores. We know that some players have legitimately exceeded these score thresholds and lose their spot on the leaderboard because of this feature. If this happened to you, we are very sorry, but you can take pride in knowing that you are one of the best. As a small consolation, the Game Over screen displays your highest score if you are interested in taking a screenshot of that and sharing it with your friends.