Because of water’s emissivity rating. Everybody knows that. 
Yes, I am well aware of the fact that a non-contact IR thermometer can be wildly inaccurate. I think my posts bear this out.
As for the assertion that someone should not be using anything IR if they can’t explain why they aren’t as accurate as other types of devices, I think that is like telling someone they should not use a wind-up watch if they can’t explain why they are not as accurate as a digital watch. Knowing that your wind-up runs 2 minutes slow a day will still get you to work on time.