The definition of the Convex Hull is based upon the perimeter, not the area:
In mathematics, the convex hull or convex envelope or convex closure of a set X of points in the Euclidean plane or in a Euclidean space (or, more generally, in an affine space over the reals) is the smallest convex set that contains X. For instance, when X is a bounded subset of the plane, the convex hull may be visualized as the shape enclosed by a rubber band stretched around X.
So this is an expected result. Although it seems to me that you have printed both the perimeter points ([0 1 2 3]) and the area (60.0)?
