Spawning in vanilla items with a customised 'armor' attribute through commands does not work. I noticed this problem in a 1.9 version and have tested it in other versions, including the latest and recommended 1.10 versions. I have tested this without any mods except Forge itself installed, and it seems to be a problem stemming from Forge.
Here is a screenshot of the problem: http://i.imgur.com/p4ZJ0ZX.png
As you can see, this iron chest plate claims it only provides the +1 defence it was spawned in with, but when worn it actually provides the full 6 defence of a vanilla chestplate. I also tried making an iron chestplate that would provide +1 defence simply if held in the off hand, and while it still claimed to give +1 defence holding it gave no defence increase at all.
This is not a purely visual issue, either. The custom items don't provide any modified defence in action. A +1 iron chestplate defends just as much as a vanilla +6 chestplate. I even ramped things up to the extreme just to make sure, by wearing a vanilla diamond chestplate and then getting hit by an iron golem, and then again wearing a custom +300 defence diamond chesplate, and there was no difference in defence.
Logs from the latest session of making and testing these custom armours, in the latest version of Forge: http://pastebin.com/MdTw0dRv
