About calibrateCamera()'s fx of standard deviation

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After calibrateCamera() function, I got intrinsic/extrinsic parameters and stdDev.
But, stdDev(fx) is always zero. Is it bug of calibrateCamera() function?
I used OpenCV 3.4.3

left camera
fx: 3,275.45, stdDev: 0.00 <<<<< always zero... why?
fy: 3275.48, stdDev: 0.44
cx: 651.28, stdDev: 0.25
cy: 440.30, stdDev: 0.46

right camera
fx: 3,261.79, stdDev: 0.00 <<<<< always zero... why?
fy: 3,261.79, stdDev: 0.43
cx: 645.59, stdDev: 0.22
cy: 427.74, stdDev: 0.44