Why is my battery drain fast in offscreen mode?

I have a Xiaomi Mi A1 phone with newest android oreo.
On first days of use, it almost didn't lose any battery charge over offscreen time , i could leave it on the shelf while running in offscreen for 3 days and its battery still had power..

After i installed some apps, the difference in battery drain is insane.
I leave the phone on the desk for like 3 hours and it loses 40% battery

I have this AccuBattery app to measure battery drain.
For example i left the phone for 18h on the desk in offscreen, didnt use it, and it drained 78%..
Used for 18h 9m - 80% to 2% (78%) - Screen time : 2m - -2.207 mAh Deep Sleep : 3min

This AccuBattery has "per app usage", but it shows nothing. It only shows like gmail used 2 mAh, maps used 3mAh, and another used 1mAh over 18h, but its nothing compared to 2207mAh.

I just want to know why does my phone drain battery in screen off mode so fast, and what can i do about it

My mother also haves a same phone, she hadnt really used it ,hasnt installed any apps, and her phone can last for days in offscreen without charging..

Would it do anything with apps in backround? (AccuBattery didnt show they use too much battery)
I have these apps, some usually send random notifications:
StopAd, AvastAntivirus, GoogleMaps, GoogleEarth, Skype, "Mechanical Engineering", some games.., voice recorder, wifi analyzer, Picai, 9gag..

Sorry for long post ,thanks if you have any idea how to help.

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