As said above, I understand that you need to designate a point near the place you want to shoot, or at least at the same elevation. Are you doing this ?
I might be really wrong but I think I understand that the Harrier only way to know the distance to target is to designate the spot you are shooting at. The system gets ranging thanks to the nose camera doing magic or the TPOD laser. So if you don't designate the point you want to shoot at, the sight can't know distance, that's why it's inaccurate.
Again, I might be talking nonsense here, I don't know a lot about the Harrier.