FWIW, I just got a Grove Sunlight Sensor today. Using the example script with the sensor connected to a Seeeduino v4, with the sensor in a closed box (no light), I see readings of about 260 for Vis, 253 for IR, and 0.02 for UV. When I take the sensor out of the box and expose it to an LED shop lamp at about 3 inches, I get readings of about Vis = 400, IR = 1100, and UV = 0.75. Just looks like it has some zero offset.