Armor pieces "overwrite" the other pieces so that only the last on put on is properly working. The other ones' effects like fire resistance or armor are gone.

I created some armor with e.g Thorns lvl. 256 and Fireresitance lvl. 256 and knockback resistance 1 and so on via /give, and put on.

The movement speed from my boots went of as I equipped my leggins, and so were the +10 armor. This appeals even with the shield (unbreakable, +1000 armor).

Reproduce:
1. /give yourself some (diamond) boots and leggings with movement speed +0.1, +10 armor protection lvl. 256, and leggings with fire resistance lvl. 256 and knockback resistance 1 (for example)
2. put the shoes on, and then the leggins, and watch the screen zoom from the movement speed go off as well as the 10 armor points disappear
3. You can try this with any armor pieces or the shield. It's not only a visual effect, but actually affecting your stats