I suspect that this is the result of some amazing historical research which proves that in battle the hoplites constant pressure to dress right and close the gaps in their ranks caused the entire unit to drift right during battle.

I first noticed it during a bridge battle where my Nubian spearmen began to shuffle right and expose the bridge threshold. The trick is not to try and correct the movement otherwise you disrupt unit cohesion.

I just move a reserve phalanx into the gap and let the Nubians shuffle themselves out of action.