关于gluUnProject的问题

Question

本人想通过gluUnProject将屏幕2维点坐标转换为3维点坐标, 但发现了问题, 为简化问题描述,将调试值写成了如下程序内容:
GLdouble modelMatrix[16];
GLdouble projMatrix[16];
GLint viewport[4];
double Xwin, Ywin, Zwin; //窗口2D坐标
double Xobj, Yobj, Zobj; //模型3D坐标
//
modelMatrix[0] = 0.00000000000000000;
modelMatrix[1] = 0.00000000000000000;
modelMatrix[2] = 1.9447331428527832 ;
modelMatrix[3] = 0.00000000000000000 ;
modelMatrix[4] = 1.9447331428527832 ;
modelMatrix[5] = 0.00000000000000000 ;
modelMatrix[6] = 0.00000000000000000 ;
modelMatrix[7] = 0.00000000000000000 ;
modelMatrix[8] = 0.00000000000000000 ;
modelMatrix[9] = 1.9447331428527832 ;
modelMatrix[10] = 0.00000000000000000 ;
modelMatrix[11] = 0.00000000000000000 ;
modelMatrix[12] = 0.00000000000000000 ;
modelMatrix[13] = 1.1748198747651376e-015 ;
modelMatrix[14] = -119.18627166748047 ;
modelMatrix[15] = 1.0000000000000000 ;
//
projMatrix[0] = 0.010926948860287666 ;
projMatrix[1] = 0.00000000000000000 ;
projMatrix[2] = 0.00000000000000000 ;
projMatrix[3] = 0.00000000000000000 ;
projMatrix[4] = 0.00000000000000000 ;
projMatrix[5] = 0.016666667535901070 ;
projMatrix[6] = 0.00000000000000000 ;
projMatrix[7] = 0.00000000000000000 ;
projMatrix[8] = 0.00000000000000000 ;
projMatrix[9] = 0.00000000000000000 ;
projMatrix[10] = -0.0020000000949949026;
projMatrix[11] = 0.00000000000000000 ;
projMatrix[12] = 0.00000000000000000 ;
projMatrix[13] = 0.00000000000000000 ;
projMatrix[14] = -1.0000000000000000 ;
projMatrix[15] = 1.0000000000000000 ;
//
viewport[0] = 0 ;
viewport[1] = 0 ;
viewport[2] = 543;
viewport[3] = 356;
//屏幕二维坐标
Xwin = 146.0;
Ywin = 36.0;
Zwin = 0.0;
//
gluUnProject(Xwin, Ywin, Zwin,modelMatrix,projMatrix,viewport, &Xobj, &Yobj, &Zobj);
CString str;
str.Format(_T("%.2lf, %.2lf, %.2lf"),Xobj,Yobj,Zobj);
AfxMessageBox(str);

//微小的变动
modelMatrix[13] = 0.00000000000000000 ;
gluUnProject(Xwin, Ywin, Zwin,modelMatrix,projMatrix,viewport, &Xobj, &Yobj, &Zobj);
//CString str;
str.Format(_T("%.2lf, %.2lf, %.2lf"),Xobj,Yobj,Zobj);
AfxMessageBox(str);

运行以上程序段发现, 当将modelMatrix[13]的值从
1.1748198747651376e-015 修改为0.00000000000000000时,
坐标(Xobj,Yobj,Zobj)的值从（1311，-22， -25）变成了（61，-22，-25）， 为什么Xobj的变化如此之大呢，本人程序是在图形中输入文本，该错误导致用户输入发生很小变化时, 文本位置发生了很大的变化，请求大家的帮助.