The online calculator only handles the case of two mediums with different refraction indicies. The situation on the sensor is more complex, as a light ray would pass through air, water ice, plastic, and then into air again on the chip side of the lens. What I think that means here is that there could be a good deal of refraction going on, with an unknown final result.
Ninchelser's teardown of the sensor showed a dark plastic foam ring around the chip. He believes it's there to prevent unwanted low angle light from getting in, and I think that is probably correct. Ice may be partially thwarting that function, or maybe the ring in your sensor is somehow not doing it's job.