There are various ways to calculate a yearly, monthly, daily and hourly average. This report averages the hourly median (not mean) PM2.5 concentration in all the stations in the city. Some outlier figures may affect the mean average produced by the government.
The government may have more monitoring stations that are either not published or that IQAir AirVisual didn’t collect. Alternatively, IQAir AirVisual may be referencing more stations for its average than the government.
Some data used in official averages may not have been collected timely and was lost.
Governments may use a different AQI or report different types of pollutants. It is important to compare PM2.5 concentration in μg/m3.