Physcial camera focal length not the same as the camera matrix focal length

[LawrenceXu13467]

Hi pykitti community,
Would anyone please explain what fu and fv here means? Typical camera focal length is <100 mm. but kitti calibration file its more than 100?
In our specific case, the camera we use has a focal length of 1.257mm physically in real life. However, the 3d bboxes do not fix to the objects until we set the focal length in the above matrix to be around 340 mm.
Does anybody know why that happens? Thanks!