Basically because actor positions are stored using floats, the further away they get from 0,0,0 in world space the more susceptible they become to jitter.
This happens because floats don’t store very large numbers with absolute accuracy, but more as approximations. So for instance, if you made a big planet mesh and centered it at 0,0,0 you’d probably find (assuming the planet was large enough) that out at the surface of the planet when you tried to move actors around they would appear to be, well, vibrating as they moved.
As it turns out, there’s a bit of a sneaky workaround for this. Just offset the whole planets position so that the part of the surface you’re standing on is at 0,0,0 in world space. That way the coordinates of the objects around you will be well within the range of values that floats can represent accurately. And with objects that are far enough away from your players position to start jittering again, the artefact will be too small to see at that distance anyway 