Why can I get more enchantment on steel armor?
- Weaker soul in a large gem. It's easy to mistake Grand Soul Gem (Great) for Grand Soul Gem (Grand).
- Obviously, any improvements to your Enchanting skill or new Enchanting perks will affect this.
- Do you have any magic effects currently active? You can't enchant a +enchant%, but you could have a potion that will affect this.
- Do you have a +restoration buff? All "Fortify X" effects are considered Restoration. Thus, a boost to your restoration will make a +enchant potion more effective than without the resto buff.
- Do you have any negative effects when you do the daedric? Aside from the obvious, I think a debuff to resto would also hamper +enchant stuff you might be using.
The material doesn't make a lick of difference. Unlike Morrowind, the item receiving the enchantment has no bearing on the magnitude of the enchantment.
AFAIK, enchantment strength is not influenced by the material of the item you are enchanting.
Are you sure that both souls used were the same strength? Note that soul gems can be filled with smaller souls than their maximum capacity, e.g. if you kill a skeever and all you have is an empty Grand Soul Gem, your Grand Soul Gem will be filled with a petty soul.