Yeah, this is because TF isn't percent but the sum of all segment's F ranges instead, see https://www.youtube.com/watch?v=hHLdW_FRtfw about Spline Mechanics for starters.
So adding a deltatimed TF to your position won't give you what you want. Try CurvySpline.MoveBy() instead, it does exactly what you want.
So adding a deltatimed TF to your position won't give you what you want. Try CurvySpline.MoveBy() instead, it does exactly what you want.