The possibilities are: harness not routed properly, especially going through the same slots in the cover but different slots in the shell; using different loops on the harness ends at the splitter plate; child sitting off-center; the harness lengths actually being substantially different.
You say you've redone it several times, so I assume that it's neither of the first two possibilities. Child being off center is common. Uneven harness lengths are extremely rare but not outside the realm of possibility. A couple of millimeters doesn't matter, but if they are different by an inch or more then it does.