Can someone explain what "Standby time" is on iOS?
All iOS use the same definitions of standby time. 

Standby is the accumulation of all of all powered on time since its lasts full charge. Whether the device is actively being used, running with the screen off or sleeping, this time counts towards the standby category. 

Usage is the amount of time the iPhone has been awake and in use since the last full charge. This use can be an active use or certain background tasks like navigation or checking mail periodically or receiving push notifications and events. 

The only time between now and the last time of charge that doesn't go towards standby is if you power down the device.

Therefore, if you've got some background tasks, such us auto-cheking emails, or any background-running Apps, the numbers for the Usage Time will increase although you're not actively using your phone.