That is not literally true. For attacks for instance, if you deal 100 damage and have +100% buff, you will deal 200 damage, 100% more. If you added another +100% attack buff, you'd deal 300 damage, or 50% more. So they give the same amount, it is just the % increase is lower. It doesn't matter whether the character attack buff comes from equipment, kami skill, or eidolan.
Defense is the same way. With +100% def a 100 damage hit becomes 50. With another +100% defense it becomes a 33 damage hit.