Forecasts need to be accurate .
The very nature of the beast is that there can be no 100% accuracy, you can just have a range of acceptable outcomes. That is you will either complain it's overly conservative and always carrying alternate fuel, or not conservative and they are missing things that actually occur. Somewhere in the middle is where forecasting sits.
That is why you are taught as a pilot to 'interpret' a forecast. The forecasts should resemble close to whats happening, and its up to the pilot to nut out the nuances.