I noticed that when the correct% value for today is greater than the value for this week, the today value gets shown in both columns. When the today correct% value is smaller, it doesn't happen.
Here's an example. The value for this week was around 90% yesterday, so there's no way it could have reached 97% with just a few extra questions:
Edit: I ran another quiz, and the today value is now 93%. The displayed value for this week is also 93%. Something is definitely going on.