I ran into a little curiosity that has me baffled. The UESP says a Giant has 591 in health. With archery enchantments, Michi's Drainspell Bow is putting out 63 in damage, including Iron Arrow. I'm playing on Novice so Michi's damage output is doubled, or 126. Four hits would total 504, 87 less than what it should take to put down a Giant, yet four hits is all it takes. I checked a couple times, making sure I wasn't scoring any critical hits and sure enough, four hits each time.

Anyone know what might be going on?

Perhaps since you are on novice, the giants also have less health then 591?

Also are you sneaking? Because if so you may be getting double damage sneak attacks

This I think, enemy health i think also scales, could be wrong though

Scaling can't be the issue. In all of the Elder Scrolls games the damage slider only increases the damage dealt by enemies and decreases the damage dealt by the player character.

My first thought was that maybe the game rounded the numbers up. But 87 is a lot to be rounding up, so I doubt that's it. Are you using any mods? Are you wearing jewelry or apparel that might be increasing your damage output? Are you drinking potions or casting spells or have any other enchantments on your character that might be affecting the numbers?

I really can't think of anything else at the moment.

Check your difficulty.

Also get some DLC and patches...