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 ...
Some explanation when something like this happens is given in this post on Mathematics Stack Exchange: Why is my total reputation less than my monthly reputation? (You might be also interested in other posts linked there, for example: What are private reputation events?)
I will quote the answer posted there:
The discrepancy is by design:
Total reputation ...