If you are using a Windows 10/8 tablet or a Windows RT-based device and you have configured the Power Options to Turn Off Display when Plugged in, to Never, then Windows may be still able to enter the Connected Standby power state and turn off the display. Microsoft has described this scenario in a Windows tablet and offered an explanation for why it happens so.
If you set Turn Off Display Power Option to Never, Windows tablet could still enter Connected Standby
In this scenario, where your system is plugged in, idle and not doing any work you may find that Windows can enter Connected Standby when you have set Turn Off Display Power Option to Never. In spite of this, the tablet will still enter the Connected Standby state and turn off the display.
Why does this happen?
KB2854058 explains that this behavior is by design.
When the machine is idle and with AC plugged in, Windows may trigger Automatic Maintenance, if it is due. During this scheduled maintenance task, winsat.exe get triggered to perform some jobs and carry out certain performance evaluation. In such scenarios, to get the work done fast and to get the accurate Windows Experience Index, winsat.exe will change the current power scheme for Highest Performance.
When this happens, the change of power scheme will change the Turn Off Display setting, to anything else, other than Never. This can trigger Connected Standby, in some cases.
Just so you know!