Another run at the GB/CB/1905 fast battlecruiser design

I have tended to be obsessed with Cliff's design for a fast British battlecruiser, what we called the GB/CB/1905 design. I just got a good result by using light weight machinery. I am mystified why the protection is so good, considering the armour basis of 4in:

GB/CB/1905, Great Britain Battlecruiser laid down 1905 (Engine 1910)

Displacement:
 23,677 t light; 24,490 t standard; 27,903 t normal; 30,632 t full load

Dimensions: Length (overall / waterline) x beam x draught (normal/deep)
 (778.00 ft / 770.00 ft) x 80.00 ft x (27.50 / 29.67 ft)
 (237.13 m / 234.70 m) x 24.38 m  x (8.38 / 9.04 m)

Armament:
      4 - 12.00" / 305 mm 45.0 cal guns - 850.00lbs / 385.55kg shells, 90 per gun
   Breech loading guns in turret on barbette mounts, 1905 Model
   2 x Twin mounts on centreline ends, evenly spread
  1 raised mount - superfiring
      9 - 6.00" / 152 mm 50.0 cal guns - 114.33lbs / 51.86kg shells, 150 per gun
   Quick firing guns in deck and hoist mounts, 1905 Model
   8 x Single mounts on sides, evenly spread
   1 x Single mount on centreline, aft deck aft
  1 raised mount
      Weight of broadside 4,429 lbs / 2,009 kg

Armour:
   - Belts:  Width (max) Length (avg)  Height (avg)
 Main: 4.00" / 102 mm 580.00 ft / 176.78 m 16.00 ft / 4.88 m
 Ends: Unarmoured
   Main Belt covers 116 % of normal length
   Main belt does not fully cover magazines and engineering spaces

   - Torpedo Bulkhead - Additional damage containing bulkheads:
  1.50" / 38 mm 580.00 ft / 176.78 m 27.00 ft / 8.23 m
 Beam between torpedo bulkheads 56.00 ft / 17.07 m

   - Gun armour: Face (max) Other gunhouse (avg) Barbette/hoist (max)
 Main: 8.00" / 203 mm 5.00" / 127 mm  8.00" / 203 mm
 2nd: 4.00" / 102 mm 3.00" / 76 mm  4.00" / 102 mm

   - Armoured deck - multiple decks:
 For and Aft decks: 3.00" / 76 mm
 Forecastle: 2.00" / 51 mm  Quarter deck: 3.00" / 76 mm

   - Conning towers: Forward 8.00" / 203 mm, Aft 5.00" / 127 mm

Machinery:
 Oil fired boilers, steam turbines, 
 Direct drive, 4 shafts, 168,353 shp / 125,591 Kw = 33.00 kts
 Range 6,500nm at 17.00 kts
 Bunker at max displacement = 6,142 tons

Complement:
 1,079 - 1,403

Cost:
 £2.236 million / $8.944 million

Distribution of weights at normal displacement:
 Armament: 923 tons, 3.3 %
    - Guns: 923 tons, 3.3 %
 Armour: 5,984 tons, 21.4 %
    - Belts: 1,501 tons, 5.4 %
    - Torpedo bulkhead: 869 tons, 3.1 %
    - Armament: 1,029 tons, 3.7 %
    - Armour Deck: 2,327 tons, 8.3 %
    - Conning Towers: 258 tons, 0.9 %
 Machinery: 6,957 tons, 24.9 %
 Hull, fittings & equipment: 9,813 tons, 35.2 %
 Fuel, ammunition & stores: 4,226 tons, 15.1 %
 Miscellaneous weights: 0 tons, 0.0 %

Overall survivability and seakeeping ability:
 Survivability (Non-critical penetrating hits needed to sink ship):
   36,559 lbs / 16,583 Kg = 42.3 x 12.0 " / 305 mm shells or 4.1 torpedoes
 Stability (Unstable if below 1.00): 1.17
 Metacentric height 4.8 ft / 1.5 m
 Roll period: 15.4 seconds
 Steadiness - As gun platform (Average = 50 %): 65 %
   - Recoil effect (Restricted arc if above 1.00): 0.55
 Seaboat quality  (Average = 1.00): 1.30

Hull form characteristics:
 Hull has low quarterdeck ,
   a ram bow and a cruiser stern
 Block coefficient (normal/deep): 0.577 / 0.587
 Length to Beam Ratio: 9.63 : 1
 'Natural speed' for length: 27.75 kts
 Power going to wave formation at top speed: 54 %
 Trim (Max stability = 0, Max steadiness = 100): 50
 Bow angle (Positive = bow angles forward): -3.00 degrees
 Stern overhang: -2.00 ft / -0.61 m
 Freeboard (% = length of deck as a percentage of waterline length):
    Fore end,  Aft end
    - Forecastle: 30.00 %,  33.00 ft / 10.06 m,  29.00 ft / 8.84 m
    - Forward deck: 30.00 %,  29.00 ft / 8.84 m,  29.00 ft / 8.84 m
    - Aft deck: 10.00 %,  29.00 ft / 8.84 m,  29.00 ft / 8.84 m
    - Quarter deck: 30.00 %,  21.00 ft / 6.40 m,  22.00 ft / 6.71 m
    - Average freeboard:  27.23 ft / 8.30 m

Ship space, strength and comments:
 Space - Hull below water (magazines/engines, low = better): 136.1 %
  - Above water (accommodation/working, high = better): 225.6 %
 Waterplane Area: 44,072 Square feet or 4,094 Square metres
 Displacement factor (Displacement / loading): 116 %
 Structure weight / hull surface area: 145 lbs/sq ft or 706 Kg/sq metre
 Hull strength (Relative):
  - Cross-sectional: 0.97
  - Longitudinal: 1.36
  - Overall: 1.00
 Cramped machinery, storage, compartmentation space
 Excellent accommodation and workspace room
 Good seaboat, rides out heavy weather easily