Stereo Calibration Large Reprojection Error

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I have two cameras of focal length around 16mm. The images are of size 1920 x 1200. The baseline of the stereo camera is 58cm.

I use a chessboard which has 11 x 8 squares and each square is of size 30mm.

I try to calibrate them. First of all, I calibrate each camera individually and the result is fine as the reprojection error is 0.1 pixel.

cameraMatrixL=:[2785.202770863591, 0, 984.1400459663615;
  0, 2788.428270564179, 581.5087637790315;
  0, 0, 1]
cameraDistcoeffL=:[-0.1677238267167475;
  -0.395313443957162;
  -0.001383372443801478;
  0.0002014464131976951;
  2.119865752288633]
cameraMatrixR=:[2609.062822974007, 0, 971.5599542214958;
  0, 2607.290530112078, 559.2017994321467;
  0, 0, 1]
cameraDistcoeffR=:[-0.2495239596557136;
  0.4314222326927977;
  0.001374092496367516;
  -0.001356964758328043;
  -0.6424581915855094]

When I use stereoCalibrate to calibrate the two cameras, with CV_CALIB_FIX_INTRINSIC flag since each camera has been calibrated, the calibration leads to very large reprojection error of about 74 pixels. And part of the result is:

R=[0.9997985779255049, -0.01978248219858788, 0.003384815534210077;
  0.01973819084128694, 0.9997251111494954, 0.01265329836216954;
  -0.003634198735761777, -0.01258393957358743, 0.9999142148526329]
T=[-541.2430345930716;
  -22.06582437757288;
  -118.2719750753764]

541mm is close to my baseline of 58cm. However, the cameras should not have a distance in other directions as large as 118mm since the two cameras are placed on the same plane.

I don't know why. Is there anyone that can provide me some adivices to improve the results?