The Phthanic Enchantment

Item of Protection +X
The of Protection spell is a very common application of magic in the basic fantasy setting. There are magical rings of protection, amulets that bestow invisible armor, and suits of armor enchanted to have more armor. The one thing that isn't explained is how this protection comes about, other than a dash of hand waving and the exaspered 'It's Magic!'

Fortification vs. Prevention
A common mode of enhancing defense is the fortification of armor, clothing, or simply the body itself. This form of magic is energy consumptive, and improved armor or not, the wearer still takes the hit. There are also unpleasant side effects of turning skin into armor. Tactile sensations are often reduced, and rigidity of the skin can make fine movements difficult.

The Phthanic Enchantment is a mix of basic magic and divination. By knowing what is coming, it is easier to avoid being injured. With a small amount of resistance, an attack can be directed to where it will do minimal damage. This magic is primal, and operates on an instinctive level. A fighter in Armor +2 doesnt have two extra seconds to guess what the enemy is doing and prepare, rather instinct and intuition, boosted by magic allow for greater evasion and deflection.

The Phthanic Enchanter
A calm and cautious magus, the master of Phthanic magics understands that haste makes waste, and a stitch in time saves nine. This care and attention to detail explains why a basic enchantment increases exponentially in cost as it's functionality increases. Weaving a minor +1 protection is something easily done. The larger the modification, the more the tapestry of reality has to be warped by the enchantment, the longer and harder the spell preparation and casting is.

The item to be enchanted can also make a spell easier or more difficult. A shield is the easiest to enchant, it is supposed to be hit with attacks. Armor and clothing are next, their main difficulty being their complex construction and multiple pieces. Most magical armors have a single major piece that is the seat of the enchantment, the rest merely enjoy it's benefit. The non-enchanted pieces are easily replaced without affecting the spell in question. Jewelry and odd objects are the most difficult to enchant as jewelry is small and the protection must come entirely from magic. A plate of steel can provide a seat to catch a strike, while a damsel wearing a magic ring can onyl deflect strikes with silken bodices and soft flesh.

In Summary
The Phthanic enchantment is a backstory element to cover the variety of magic items that provide bonuses to avoid injury in combat. Rather than taking and ignoring hits, the beneficiary of a Phthanic enchantment is more difficult to hit, be it with a sword, or an arrow.