There are several reasons for this to happen:
1- The air quality may vary greatly even within a few miles/km.
2- The official government stations usually report measurement as an average of the past hour. In practice, it means the official government data reported might already be 1 to 2 hours old.
3- Frequency of the update: some government stations refresh several hours late. By the time the data is published, it is already outdated by a few hours. The Node/Pro has the advantage to show real-time data.
4- The government station doesn't actually report PM2.5. In this case, the Node/Pro would display a note below the outdoor data "Main pollutant is xyz".
5- The Node/Pro or the government station may actually show an inaccurate measurement. Government stations require frequent and expensive maintenance which is not always dispensed in time.