This statistic appears to be programmed to display the highest rank available between weekly, monthly, quarterly, yearly and all-time (with a bias towards the longer-term in case of ties). You can check by clicking on the profiles of the leaders on the reputation leagues.
It could be one or two things:
caching - reputation leagues are updated once a day, the /users page more often.
The 'reputation change' column in the reputation league includes reputation from so-called private reputation events: see Why is my total reputation less than my monthly reputation? on Meta Stack Exchange. For example, reputation gained on answers ...