We have looked into this a bit more and the figures appear to differ as they are reporting the air quality index which doesn’t correspond exactly to the concentration figure measured by the sensors. If anyone doesn’t know this report describes how to calculate it https://www.airnow.gov/sites/default/files/2020-05/aqi-technical-assistance-document-sept2018.pdf
A very quick look at the information suggests that they are using the data directly from the sensor community output but are not using spikes. I am not sure how long they have been adding the data to their map but it would be good if they produced an archive of the information such as they do for the regulated sites.
I would be interested in knowing if they are collaborating in anyway with the sensor community.