Grip value varies between sources

It is normal that the connected car data grip displays something slightly different from what is measured by road weather stations or what is estimated by the network assessment.

The sensors measure the impact of the weather on the road surface. The connected car data indicates the experience of the vehicle on the road surface that is impacted by the weather, and that itself is highly determined by the equipment on the vehicle. The data set Wx Horizon gets is implicitly from the newest vehicles on the network. They are well equipped and may be using winter tires. While a weather station might indicate that there is a water film thickness or a certain level of ice on the road, if you are in a vehicle driving with winter tires made for this weather, the experience will be different.