Weapon Pop Up / Comparison in Inventory

Noob question here. When I look at my inventory and move a weapon to a different slot, a window shows the weapon as Current and New. What does this actually represent and why are some of the stats in the New column sometimes red? Thanks in advance.
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As an example, I have Plate mail equipped (AC 3), and I select splint mail (AC 4), it shows the current AC and the AC if equipping the selected item. Since plate is better than splint, it's red:
Otherwise, if I have splint equipped and I select plate, since it's an improvement, it shows the new AC in green:
Note that those stats are the final character stats (not the gear itself), so if I also have a medium shield (AC +1), it will show the resulting AC (in this case, plate with final AC 2, and splint with final AC 3):