when i use the evaluate curve component i noticed that the relevant length in the specific point is not the right percentage of the total length of the curve (in this example the curve is a polyline). Why is that?
curve parameters do not have a guaranteed linear progression along the curve. Lines, Arc and Circles typically do, but Nurbs curves or polylines do not. You can see this by feeding a Range component into the Eval curve component and create -say- 100 points with equal parameter spacing.
If you want to evaluate based on length then you should use the [Evaluate Length] component.