Why can 3rd party apps still use background mobile data when it's tunred off?

[Android Central Question]

I have toggled off mobile data, background data & roaming. I also have data saver turned on.

Yet when using an app today (with mobile data turned off but calls etc on) I noticed it appeared to have got some information online. I went to check past week, 24h etc to find it and other apps had managed to use background data despite the settings supposedly preventing this. I can understand (however annoying) for some android apps they may be part of the OS and despite me setting off at an app level there being a way this is overwritten but I cannot understand why this would be the case for 3rd party apps.
I no longer have WiFi at home so have been paying more attention to what is happening with my mobile data, this may have been happening for a while.

Honor 9 Lite EMUI 8.0.0.144 updated April 2019
Android 8 kernal version 4.4. 23+ updated 26th March. Security patch 5th April.

Apps I noticed it on is:
Pulse by Camera
Greenify
A game which I have uninstalled

Sheets
Google

(I don't know if its related but my downloads have started being duplicated)

I have run the built in avast virus scan

 

[Rukbat]

If you have picture sync enabled (the default), that may be using data to sync the pictures. Google (the app you can ask questions to) always checks the web for answers. Sheets? I don't know, I never used it.

Sync and Google are system apps, so they may be exempt from the "don't use background data" setting.