Why does it seem like the device time limit reaches the usage quota sooner than the actual app usages on the child's device?
Total device time is the amount of time that the device screen display has been visible (i.e. not switched off). This includes total app usage time (time spent running applications) and also any time that the device was idle but the display was still visible.
It is possible that the child's device has a long screen timeout in the Android's Setting so that the screen stays on even if it is not being used. Hence the device screen display is counted toward the total device time and leaves a smaller portion to the actual app usages on the device.
To minimize the device screen display time (while in idle) being counted in the device time limit quota, parents can either adjust the 'screen timeout' in the Android Settings of the child device to a shorter duration (Andriod Settings -> Display -> Screen Timeout), or simply inform the child to 'screen off' the device when not used.