Difference between contour data and station data?

[tnordberg]

Hello, thank you for providing the excellent AirNow data API.

I am working on building a museum exhibit about air quality based on the AirNow data API. A significant part of the exhibit is creating an animated air-quality contour map, and so I have been trying to understand how the AirNow contour map works. I have found one aspect of it confusing and suspect it may represent a bug; if not, it would be very helpful to have an explanation of how these contours are generated!

I noticed that some monitoring stations are placed on contours that do not seem to match their value, such as this yellow (moderate) station placed in a green (good) contour:

I then noticed that, if i expanded the "Plot Data" option, the indicated AQI of 53 (which is a yellow/moderate value) does not apparently match with the current reading of 46 (a green/good value).

However, four hours prior, the reading was 53!

This seemed to suggest that the contours might be based on the current AQI readings, while the monitor station values were displayed by their values as of four hours ago. I tried some other stations, which seemed to yield the same results, even for stations whose AQI category matched the contour category.

Is this a bug? Am I understanding how these contours are generated correctly? Any feedback you can give would be greatly appreciated!

Thank you!