Something wrong with gmx traj command

GROMACS version: 2021.2
GROMACS modification: No
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I want to extract z coordinate of atoms using gmx traj. With -com option I was expecting the COM position to be average of individual z-positions, but it is coming out to be higher (below figure, red curve). I tested gmx traj command on 2021.2 and 2018.5 version, in both cases getting the same result.

Any suggestions?