It's because of the trigger. Well actually it's you, but the trigger ain't helping. You are mashing the trigger and/or flinching to get them grouping like that. A heavy and/or long trigger, especially one that stacks, makes this a tough challenge to overcome, especially if you happen to be a new shooter who is trying to develop proper technique. The best remedy is to dry fire the heck out of it, and when you do, you need to concentrate on two things:
1) Focusing on the front sight.
2) Gradually pulling the trigger straight back. It should surprise you when it breaks.
This goes for live fire too. In live fire, go slow 'till you get this remedied. Breathe and take each shot one at a time, concentrating on front sight and the "surprise break" that Jeff Cooper describes.
It will improve.
Save up for a 1911 or something with a better trigger. I'm not trashing the Sigma, it fills a necessary niche in the gun world. The Sigma can be shot accurately, but it is not a conducive platform for learning the fundamentals IMHO. They are not "junk", but the trigger is just not good IME.
Jason