pretty sure it has to do with rounding. There are some that will give you a +5000 or whatever, but generally they add no extra value.

The rounding of value is quite confusing and not so great.

If the added value does not reach a certain amount, then you get nothing.

I crafted an Epic Opulent Heaume and tried enchting it with an Epic Corrupt Element. I saw that the base values stays the same. I tried a flawless Wyrmblood Ointment with a flawless Enchant and still no value. 

But Green and Normal Enchants (Tier 4) add value to green and normal items? 

Enchanting Legendary Gear with T4 also adds value. I don't quite understand the system.

Why do Epic and Flawless T4 Entchants not add value?