Outside of the valid range of BSpline Curve and Surface

I created a spline curve which is defined in range [0, 1].
However, I can get a result with u outside the valid range.

#include <TColgp_Array1OfPnt.hxx>
#include <gp_Pnt.hxx>
#include <TColStd_Array1OfReal.hxx>
#include <TColStd_Array1OfInteger.hxx>
#include <Geom_BSplineCurve.hxx>
#include <Geom_Plane.hxx>
#include <GeomAPI_IntCS.hxx>

void test_07()
{
TColgp_Array1OfPnt poles(1, 2);
poles[1] = gp_Pnt(0, 0, 0);
poles[2] = gp_Pnt(1, 1, 1);
TColStd_Array1OfReal knots(1, 2);
knots[1] = 0;
knots[2] = 1;
TColStd_Array1OfInteger multiplicities(1, 2);
multiplicities[1] = 2;
multiplicities[2] = 2;
Standard_Integer degree = 1;
Handle(Geom_BSplineCurve) curve = new Geom_BSplineCurve(poles, knots, multiplicities, degree, false);
double u1 = curve->FirstParameter();
double u2 = curve->LastParameter();
gp_Pnt p1 = curve->Value(0);
gp_Pnt p2 = curve->Value(0.5);
gp_Pnt p3 = curve->Value(1);
gp_Pnt p4 = curve->Value(1.1);
gp_Pnt p5 = curve->Value(-0.1);
}

What is this?