I imagine this inaccuracy is because the nearest stations are over four hours away. I also imagine four hours to the nearest station is common for a large portion of the globe. I suspect this difference in forecasted temperature and the actual temperature is increasingly large the further back you go in time.
It seems like there is a lot of noise and dirty data to be so confident about a couple degree increase.
Basically, the concept of “What temperature is it outside my house?” And “How much have temperatures raised around the globe?” Are wildly different. The fact that the units are the same is misleading.