I believe I have the answer!
I looked inside the light and observed that both of the adjusters connect to 1 side of the projector rather than centrally.
So, if you turn the top adjuster to pull the beam up, presumably it moves the connecting point of the projector/adjuster (captive nut on threaded rod?) back and also causes some lateral movement as this point is off the centre line (also twisting the beam so it is no longer level).
If you then correct it with the lower adjuster, pulling the beam back down also moving the connecting point back (and re-levelling the beam), you have effected lateral adjustment.
I just need some darkness now to confirm!