The mean wind in a layer from surface to 450ft height above ground level.
With the slider at 6400', you see winds at a layer aprox. 6800 ft.
Generally model's winds for your location (elevation 5800') are 400ft off.
As you go upwards, this difference decreases.
About grey lines, please read https://community.windy.com/topic/6998/windy-offers-sounding-forecast/32