Yeah it doesn't make sense. For eg. My iPad over the last 24 hours shows 15 minutes of Screen Off time.
Pretty sure the screen off time, just like screen on time, is related to usage--basically it's usage with the screen on and usage with screen off (something wrapping up in the background, etc.).
Screen off time isn’t supposed to be a counter of the time the screen is off but rather the time the device is working with the screen off. With that said, I’m not convinced any of this is terribly accurate either.
Pretty sure the screen off time, just like screen on time, is related to usage--basically it's usage with the screen on and usage with screen off (something wrapping up in the background, etc.).