It was raining all day, but the API return 0mm for the precip_today_metric while still returning the rain icon. Why?

A few days ago, it was raining in Montreal. The API knew this because the JSON response had the "rain" icon in it. However, every metric related to precipitation was returning 0.0mm, except for the precip_1_hr_in which was -9999 (null).

My question is how can the API return the right icon but not the right amount of precipitation? I was able to confirm that it was raining by simply looking outside the window.