Well, it is averaging so there is lots of inaccuracy.
For 10 days mine shows 9h 6m on, 5h 1m off, so the discrepancy is bigger.
None of my days go to the top of my graph, which is 21 hours for me, not 24. You have 3 days that go 24 hours, 4 that are close, and 3 that are not close. Maybe there is "charge time" that is not in these numbers. Screen is off and no activity is happening. In your 3 24 hour days, were you sleeping to music? That could account for those days.
Mine has 8 bars (9 if you count today) that have a 7 hour gap of nothing at the top of the graph. Only 2 days where the phone was active for 20 hours. That could be my Do Not Disturb time.
No I wasn't sleeping to any music, I don't turn my phone off either which could account for some discrepancies.
I think I was just reading it incorrectly and thought it was measuring both active and inactive timed for a 24 hour period each day.
Thanks for checking that, I couldn't find anything myself either.
I'd have thought the inactive time would still show but who knows.
It could just sort itself out. I had a problem with my screen time not showing a full week which suddenly started working apparently on its own.