No insult intended, but how much experience do any of you have shooting a DA-only design like the Sigma? Many, many people will shoot low-left when they first shoot such a gun, and it usually is not the sights; rather, it is improper placement of the trigger finger combined perhaps with a failure to stay focused on the front sight and hold a clean follow-through. In other words, just a bit of an anticipatory flinch, not even noticieable.
This is very good practice for any DA fire with pistol or revolver. Try dryfiring as a diagnostic tool. Aim at a spot on the wall (EMPTY GUN!!!). Pay strict attention to the front sight and "fire" a round. DO NOT LET YOUR FOCUS MOVE FROM THE FRONT SIGHT -- and I will bet you a dollar that the sight does down-left. Adjust your grip and finger placement on the trigger, and continue until this movement disappears. Next time you live-fire, relax, let muscle memory take over, and fire away.
Now, this may not be your problem at all, but I have "cured" the same "problem" with about 10 folks so far, with 'nary a sight being moved or changed out.