Tangents at the endpoints

Please explain me. If I make curve with interpolated points, by example, I set 4 points. Rhino build curve with 6 control points. Two endpoints coincide with interpolated points and 2 following additional points located on the tangent at the end points. What algorithm determines the direction of tangents Rnino in this case? I tried to draw a tangent to the local Bezier curve - they do not match. It’s not a secret?

Hi renegator- you should probably check with the developers here - http://discourse.mcneel.com/c/rhino-developer