Diffrence between ParamSrfIso ( Pufferfish ) position and Evaluate Curve

Hi there.
Yes, I guess I can’t understand everything but… If somebody could explain this difference between Parameter Surface Iso Curve from Pufferfish and and a simple Eval…

I thought that a parametrized curve or a parametrized surface could give the same position of a “parameter” ? No ?

I share my definition of this small test if you have a explanation… And a solution ?
Test-Angles.gh (8.5 KB)