The tooltip should indicate the relevant date range.
It indicates a specific day, which corresponds to the 'Days' tab.
Screenshots

Reported in https://wordpress.org/support/topic/tooltips-on-stats-pages-wrong/
3622193-zen
I'm working on a potential fix for this in D55329-code.
This bug was caused by a problem with the script coming from WPCOM, and we've fixed the script in D55329-code. This bug should now be fixed, so I'm closing this issue.
Let us know if you encounter any problems!
Most helpful comment
This bug was caused by a problem with the script coming from WPCOM, and we've fixed the script in D55329-code. This bug should now be fixed, so I'm closing this issue.
Let us know if you encounter any problems!