I've had that happen a lot 😂 I might have an explanation for you. I have an automation set up that has Siri read me the weather and upcoming events as soon as my alarm is stopped in the morning. When it speaks the temperature, it doesn't just say a whole, rounded number. For instance, a few weeks ago, the app said -0, but Siri read the temperature as "negative 0.8 degrees Fahrenheit." And then another morning, it showed as 0 without the negative, and Siri read the temperature as something like "0.5 degrees Fahrenheit".
The detail of "tenths of a degree" are in fact included in the weather data, but just "hidden" for the sake of showing a whole number. Just have to do a bit of digging to find it!
Cool, thanks for the explanation.