How is Screen Time calculated? Why is the calculated time seemingly longer than time spent on iPhone?
This is a bug on ios 14.
From an article on Medium.com:

...Turns out I’m not the only one seeing weird issues with screentime.
There doesn’t seem to be a correlation in regards to the iPhone model.
It also doesn’t seem to be a very common problem. However, others have
been reporting other odd screentime glitches. Some have reported their
screentime being inaccurately high with no app to blame. And others
are also reporting that their phone claims they’ve spent hours on
websites they’ve never visited.


My Screen Time claims that I was using my iPhone for 24Hours! (let's hope I wasn't...).

I have visited stackoverflow for about 1 minute, however, it's calculated as 24 hours.

Also, all apps were fully closed, no apps were running in the background.