Nolio it might seem that way at first, but it does increase their deaths at 2X the rate. They go from a base 20 down to a base 10 defense, so when they receive their allocation of damages it ends up killing twice as many units. The SB sent against you had the True Silver buff and is also buffed by the Plate Armor tech as well (I think, maybe), so they wouldn't have been quite as easy to kill as some other kinds of units that have lower defenses - but still they were twice as easy to kill technically.
I noticed what part of the discrepancy in your numbers was, it's due to the higher tier of Castle Based Defense Scaling you reached. You actually defended with only 70% of your forces, not 80%. That was a small bug though, because you should have been in the tier for 75% defense instead. I updated the logic for that to properly reflect the defense forces in the 20-29 castle tier. So with 20% healing, it makes sense you'd have around half your forces alive.
The final battle numbers looked like this:
Melee Phase
Attacker damage points: 40k
Defender damage points: 16k
Attacker Losses: 1: 963, 2: 250, 3: 262, 4: 23, 5: 163, 6: 296.
Defender Losses: 1: 876, 2: 0, 3: 0, 4: 0, 5: 0, 6: 1087.
Short phase
Attacker damage points: 223
Defender damage points: 0
Attacker Losses: 1: 0, 2: 0, 3: 0, 4: 0, 5: 0, 6: 0.
Defender Losses: 1: 4, 2: 0, 3: 0, 4: 0, 5: 0, 6: 6.
Range phase
Attacker damage points: 77k
Defender damage points: 85k
Attacker Losses: 1: 1811, 2: 1757, 3: 982, 4: 527, 5: 2800, 6: 1052.
Defender Losses: 1: 500, 2: 5982, 3: 7615, 4: 2165, 5: 7662, 6: 853.
Total Attacker Melee Points: 72k
Total Attacker Defense Points: 230k
Total Attacker Ranged Points: 77k
Total Defender Melee Points: 90k
Total Defender Defense Points: 152k
Total Defender Ranged Points: 85k
You can see in those numbers, if their ranged attack hadn't been buffed so much then a bunch of your 90k worth of melee attack could have survived into the Melee round to be able to kill more of the attackers units and also absorb damages for your cats/bals. Since the fodder units were wiped at Range, it left the cats/bals pretty defenseless against what was left of the formidable Dwarf army.
