If the new item has a stat that the old item doesn't have, it doesn't indicate that, or it's ambiguous.
For example, if I'm wearing a hypothetical item with +8 Ranged.
If I compare an item with +8 Sorcery, the item will show
+8 Sorcery
Ranged -8 (in red, indicating a stat loss)
However, if I now assume I'm wearing an identical item with +8 Sorcery, the item comparision will show
+8 Sorcery.
If there's no stat difference, or a completely new stat, the comparison shows identically.
Basically, if there's a completely new stat, it should show the parenthetical bonus, like
+8 Sorcery (+8)
rather than no modifier.