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We follow the pattern set by NOAA (which can generally be assumed for both Mathematica and Wolfram|Alpha).
WindSpeed is the wind speed at the time the weather station reported in.
MaxWindSpeed corresponds to the highest wind speed over a time period and thus is only available for periods of Day, Month or Year.
MeanWindSpeed corresponds to a mean of all wind speeds over a time period and thus is only available for periods of Day, Month or Year.
WindGusts when present follow the NOAA definition: "Is a sudden, brief increase in speed of the wind. According to U.S. weather observing practice, gusts are reported when the peak wind speed reaches at least 16 knots and the variation in wind speed between the peaks and lulls is at least 9 knots. The duration of a gust is usually less than 20 seconds."
One thing to keep in mind is that NOAA, the primary source for weather data, does not do much in the way of curation of this data. We cannot guarantee that every weather station follows the same protocols or returns data reliably.