Having one of the beam mounts too far forward or backward would indeed only cause an issue on the toe setting. But 4 mm toe in on one side and 0 mm on the other isn't much. To cause that the beam would only need to move about 4 mm forward/backward on one side (if I didn't mess up with the calculations). If the beam was really out by 2 cm the tracking should have been about 10 mm instead of 4 mm.

If the beam mounting points are also that much out it should be clearly visible that one wheel is further back in the arch than the other.