Screen time and idle time not accurate

Screen Idle includes background processes that apps are using. Some think that Screen Active and Screen Idle means Screen On and Screen Off, but that is not what it is. Previously it was listed as Screen Off, but Apple changed the wording to make it more accurate when related to the usage of your battery when viewed in Settings. The AOD (Always On Display) added another wrinkle to the Screen On/Off wording.

Some apps make significant user of Background Processes that affect battery usage with some of the unexpected ones being Meta apps such as Facebook or Instagram. Others are much more clear such as a Music app that is playing in the background.

Brklyn-Gal wrote:

I can be off my device for several hours, and time idle shows only under 30 or less minutes.

That is because “idle time” does not mean “time you didn’t touch the phone.”

On an iPhone, you can be away from the device for several hours, yet Screen Time may show less than 30 minutes of idle time. This happens because the iPhone continues running background processes—such as app refresh, iCloud syncing, notifications, location checks, and network activity—even when the screen is off. These background activities don’t count as active usage, but they can prevent the device from being recorded as fully idle for long stretches.

Turning off Background App Refresh only affects updating the UI of the app so it is correctly represented when you open the app, it has not effect on the Background processes that an app is able to perform. When an app receives a Notification the app will open to run the payload included in the notification and that is done in the background.

Normally the app is just suspended in the background, but if you force closed the app, it then has to launch and will consume more battery to do that. Force closing an app does not prevent it from receiving notifications and the background process will continue to run. If that was not the case, the only time you would receive a Notification is if an app is suspended in the background after being previously opened.

This may help --> Understanding Screen Time Graph and Calculation… - Apple Community