The 'Damage given per STR required' table is only illustrating one factor, and there are other factors to read when determining swords balance.
If you check my Shirokata post, I also have that same chart. Is that the only thing I used to convey my point? To check a sword's value in Nin, you'd look at 3 attributes (there might be more than 3, but we'll get there):
- Damage given per STR required
- Attack speed
- Any other unique benefit
You can't just compared the first attribute and conclude a sword is weak. Bone sword has drastically lower damage than Blood Katana (another sword in its tier) because of the difference in attack speed.
With your approach, Blood Katana is the best sword in the game, and Dark Bandit Blade is also really good, right behind Bone, which we know isn't true...
The low-STR swords have very little unique benefit (attribute #3) and poor attack speed (attribute #2). So you see their first attribute being their strongest.
The high-STR swords (Blood, Bone, Kyuketsuki) have faster attack speed than the low-STR (except Blood), with Religious as an exception. They also have HP regen (aka, the unique benefit). Blood Katana doesn't have HP regen or fast attack, but its first attribute (Damage) is its best.
So you notice how all 3 attributes are considered in a sword, not just the first attribute.