There cannot be any 0.5s in items. Only 1 or 2.
Certain items that are upgraded to be more powerful/stronger/etc can go from +1 to +2 in stats. IE, you can improve upon one item. So if you have a sword (+1 offensive) and you make it magic somehow, it can now be a +2 offensive sword. And now, your +2 offensive is maxed out.