I was mass-enchanting some false Harbinger gloves hoping for a specific enchantment when the shield gem on the False Harbinger gloves I’ve had for a while infused onto all of them without consuming gems from my inventory. The effect was also exactly the same (in %).
As this occurred on gloves, it is safe to assume it can happen to armour but more testing will need to be done to see if weapons are affected as well.
The specific conditions are also unclear as I have been unable to replicate this.