While the device is water resistant, it isn't completely waterproof.
The cavity under the sound holes has a series of baffles and compartments to direct water away from the temperature and humidity sensor which is inside this area. The rest of the board is sealed off from the area under the sound holes by a rubber gasket. The baffles direct any water that gets into the sound hole to a drain hole that drains out of the bottom. This will protect it from water dripping on to the unit.
However, if a directed jet of water were to shoot directly into the sound holes, it could quickly fill the sealed cavity before the drain hole could drain it all. This could lead to water getting into the temperature and humidity sensor, which is likely to permanently damage it. This sensor needs to be exposed to the outside environment to get an accurate reading so it is impossible to 100% protect it from water damage and still be accurate.