There can be 2 reasons.
1. For each challenge the users can form different teams. The rating that's shown is the sum of ratings that a user has got from all challenges in the season. The current team members may not be the team members for all the challenges
2. We give the rating reward for the current challenge, when the challenge is going on as well and not just at the end of the challenge. During the challenge the ratings for the users can increase/decrease based the performance of all other teams. For the users who're playing the game the ratings get updated based on their team rank immediately. For the users who're offline the ratings get updated when they launch the game. At the end of challenge all the users in the team get the same rating.