Hey I have a question, why degrade the values ??of objects when enchanting?

For example, shoes with 44 arms I want to enchant with +20% damage 1hand.
Question 1: Why are only +15 at the end out of it, even if I take one mighty stone?
Question 2: Why has the shoe after only 23 armor?

The same happens also with weapons, if I enchant + fire damage, only about 2 / 3 of base damage are left?

Mfg.