Richard: remember, that in the Northern Hemisphere, when a wind gusts it tends to veer ie. increase its direction in a clockwise direction. This means that with a crosswind from the left then the gust will veer and make the crosswind component less. The converse is true from the right! By the way, this is not an exact science either in the measuring or reporting.
To visualise this easily - imagine a crosswind that is at exactly 90 degrees to the runway. This results in only drift but no change in groundspeed. Now imagine the effect of a veering gust.