Screen Time on iPhone 8 and iPad shows wrong total device time

My child uses an Iphone 8 - ios 16.xx and an Ipad 2020 -ios 17.xx

Screen Time > Child Family Name > View All and Website Activity > Devices > All Devices

Time all devices 1h, 35

Iphone 1h, 35

iPad 1h, 23

I think something is going wrong here, arithmetically I come to: Time all devices on 2h, 58

What's wrong here?

[Re-Titled by Moderator]

Original Title: Ios parental control | device total time | not added correctly