Models used by Windy provide forecasts for much more vertical levels than available on UI level scale.
E.g. GFS 0.25 provides values of u/v wind components at [10, 20, 50, 80, 100, ...] meters above ground (48 vertical levels in all for wind); I'm not sure all of them would be of a general interest, thus I guess Windy made a selection of levels on UI.

About the remark "When I change the altitude in scale the forecast changes only for present time.": nope, currently selected level un UI correctly follows all available forecasts (even on animations).