The Magic Shield gives an additional +8 magic but decreases -5 HP. Why doesn't it show it on the "POSSESION" space? It only shows the defense benefits not the downgrades and other upgrades...
It's a limitation of the engine. Weapons only show the ATK stat as well. I went and got a plugin from Yanfly in order to show the other stats. You will likely have to do the same if you want to show all the other stat gains/losses.
Or use SRD's Equip Upgrade Comparison. It allows for showing all increases and decreases, you can set it up so that it shows not only base parameter changes but, sp & ex parameters, elemental rates, and so much more.