I finally found a good example, we have snow in the forecast. In the top half, the units are metric, bottom half Imperial. We are supposed to get 2.3" of snow on Dec 11, or 5.8 cm. However, WU is showing 5.8 mm (0.58 cm).
So the issue is that in Imperial, the unit between rain and snow is the same, however the unit is different in metric, so they convert, but forget to update the unit to reflect the conversion.