why the camera's parameters are not right?

I use the opengl to make a virtual camera, I put the object(it is a plane with texture on it),and I use the glRotatef( ) and the glRotatef( ) function to change the object’s position, after some move, i get several pictures with the known parameters, after that i calibrated the camera’s intrinsic and extrinsic parameters using those get picture,but most of the result are not accord with the know paremeters,any one can tell me why?