LondonFA:
Search the forums for visceral fat vs sub cutaneous fat.
That hard round belly (other than genetic influence) will come with a mostly visceral gain. Note there are greater correlated health complications related to it.
In essence, far around the organs (and hence below the abdominal wall) make for a more rounded belly shape. However, this type of fat is supposed to affect number of biological processes including insulin regulation hence the claimed link between obesity and type 2 diabetes.
becomingoverweight:
Single belly doesn't necessarily mean a ball belly. It could apply to a soft belly which lacks the large, persistent crease around the belly button which characterizes double bellies.
Indeed, hence I said aside from genetics. But on that point it is worth stating that there doesn’t seem to be much you can do to prevent a double belly forming if you’re predetermined by genetics toward it.
If you’re looking for a nice subcutaneous gain there’s loads of advice on the forums, however, so far, most gainers I’ve conversed with on here, don’t seem to have found a way to affect where they put fat on aside from aiming their diet at either a visceral or a SC gain [there is obviously the use of hormone therapies and some evidence of posterior shape change from sitting a lot].