I think these are the two lines (from ItemCombos.txt) that are causing what you're seeing, Felix:
use category Anvil on object LOOT_OreBar_A_Iron_A_Large with transform code Transform2Boost2 parameters WPN_Crafted2HSword_05,0,0,Weapon_Large_Damage_Mod,0,0,1,1 requires Blacksmithing 5 autolevel 1
use category Anvil on object LOOT_OreBar_A_Iron_A_Large with transform code Transform2Boost2 parameters WPN_Crafted2HSword_05,0,0,Weapon_Small_Strength_ModSword,0,0,1,1 requires Blacksmithing 5 autolevel 1
Basically, if you have Blacksmithing 5 and toss a Large Iron Bar onto the Anvil, one of two things can happen:
1) it will make a Crafted Two Handed Sword #5 and add a Large Damage Boost to it
2) it will make a Crafted Two Handed Sword #5 and add a Small Strength Boost to it
Either way it uses the base damage (autoleveled to the crafter's level) and then applies the boost. If it gets the Large Damage it's going to do more damage than if its gets the Small Strength.
Make sense? I haven't tested this specifically but I've done tons of mods to the crafting file (ItemCombos.txt) and this is what it appears to be programmed to happen.
Also, checking the weapons.txt file (the one under Stats\Generated\Data) shows that the sword created with Blacksmithing 4 actually does 5% MORE damage than the one created with Blacksmithing 5 (DamageBoost -15 on #4 vs. DamageBoost -20 on #5 both with same base damage of 5).
