Are you using a custom driver, by chance? If so, maybe a link to it?
Or maybe the data reported back is somehow region/location specific? Just a guess...
Perhaps you're using this driver, which makes an educated guess/calculation of illuminance...???
Personally, I use @iharyadi's awesome Environment sensor for most of my weather data. It is zigbee and provides temperature, humidity, pressure, and illuminance. I have one plugged in on my screen-porch. It works great!
