The Spline modeler creates mathematical "fake" geometry; that is, it has dynamic, non-destructive fidelity and manipulation capabilities. (this is like hyper nurbs in other 3D applications or when you apply dynamic smoothing on a vertex object in the Vertex modeling room)

While the preview may show a truncated version, the object when rendered will not. You can control the render-time fidelity by going to Geometry => Surface Fidelity. So, it may look lower resolution than it actually renders. 

Increasing the Spline surface fidelity WILL increase render-time fidelity and controls the amount of polygons if you convert or export the spline object .

Mark