We currently have two weekly leaderboards for wins and kills. We've noticed in recent weeks that when leaderboards reset, some players are still incrementing the previous week's value instead of incrementing from 0. These players seem to be carrying their previous week's value over to the next week's leaderboards. We dealt with this so far by manually resetting weekly leaderboards.
Regarding updating player statistics: We have already taken into account players who may be playing during a leaderboard reset so this shouldn't be an issue (set player statistic is called in our cloud scripts and retrieve current player statistics before updating to make sure it has the most up to date statistics).