GLUquadricObj display

OpenGL OpenGL: Basic Coding
1

The code is like following:

/**********************************/
GLUquadricObj *Bubble;
Bubble=gluNewQuadric();
gluQuadricDrawStyle(Bubble, GLU_FILL);
gluQuadricTexture(Bubble, true);
gluQuadricCallback(Bubble, GLU_ERROR, ErrorCallback);
float radius[4]={2.0, 2.0, 2.0, 2.0};

StartList=glGenLists(4);	

glNewList(StartList, GL_COMPILE);
	gluSphere(Bubble, radius[0], 10, 10);
glEndList();

glNewList(StartList+1, GL_COMPILE);
	gluSphere(Bubble, radius[1], 10, 10);
glEndList();

glNewList(StartList+2, GL_COMPILE);
	gluSphere(Bubble, radius[2], 10, 10);
glEndList();

glNewList(StartList+3, GL_COMPILE);
	gluSphere(Bubble, radius[3], 10, 10);
glEndList();

/**********************************/

To my surprise, I found that bubbles flickered greatly when the four components of the “radius” are different. But when they are the same, say 2.1, they nearly don’t flicker. Why and how to solve it?
Thanks in advance.