Point Cloud stretched in one dimension

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I have calibrated a stereo camera using this, and I am getting this point cloud:

The problem is that in the point cloud the squares are not squares but rectangles (while in the image they are squares). In other words, the width of the board is the same (in the point cloud and in the image), but the height of the board is bigger in the point cloud.

I have also tried to calibrate it again, but the result is the same. How could I solve this?

Thanks