You're seeing the usage of a single client against the usage of all clients that match the current filter criteria for the summary report.
Here's a sample from a smaller network where you can do the math by hand. If I change the filter to show a single network or use a different date range, both the percentages and totals change as you would expect.
![Meraki_summary.png Meraki_summary.png](https://community.meraki.com/t5/image/serverpage/image-id/828i9D9ED65226664A22/image-size/large?v=v2&px=999)