CameraCalibration only stores 3 factors independent of setting

[donmahallem]

Inside the CameraCalibration Node on can set the "Nb Distortion to Coef" to anything from 1-5 but https://github.com/alicevision/AliceVision/blob/c1f79a9d20e4c2b18d23d3d548d03a6c0e892213/src/aliceVision/calibration/exportData.cpp#LL140C2-L151C4 only store the coefficients 1,2 and 4 described as k1,k2,k3. is this on purpose? And if so can anyone explain to me why?