When you see a report that the wind speed at the airport is, for example, 15 miles per hour, it would be unusual for the wind speed to actually be steady at that speed. Officially, the so-called sustained wind is actually the averaged wind speed over a period of 2 minutes. It is only when there is a brief and sudden increase of the wind of at least 10 mph over that averaged wind that a gust is reported.
Therefore on most occasions the wind does not blow at a steady pace, but instead blows constantly at slightly differing speeds and often blows in a series of gusts of differing magnitudes.
There are many reasons the air motion in the atmosphere is not steady, but the most common are, differential heating, which creates thermals and uneven mixing of the air and that the terrain is not uniform and therefore the wind movement is not allowed to flow evenly over the surface of the earth.