The whole business of scaling AC by level, without typing it, seemed off to me.
The problem appears in 3/3.5E, and goes back to 1E, with the scaling of AC by HD.
Using demons as an example, the table goes:
Hit Dice, Challenge Rating, Natural Armor Bonus
HD 2 CR 2 NAB 5
HD 7 CR 6 NAB 8
HD 12 CR 10 NAB 13
HD 12 CR 13 NAB 19
HD 17 CR 17 NAB 16
HD 20 CR 20 NAB 19
With full plate providing an armor bonus of 8, that gives the higher level demons the equivalent of 2 to 2.5 the value of full plate.
(There was a similar effect with drow and their very high armor plus values, +5 bucklers, +5 armor, and +5 rings of protection adding up to +15 to AC.)
The reasoning is given (I believe) as their otherworldly nature, which I can accept for a while, but the very high values begin to seem absurd.
That the game has bared progressions (+1 level give +1 attack bonus, +1 level gives +1 AC, or some multiple there-of) seems to degrade the narrative of the game. This is a residue of the original game design which was fun for a little while, but now seems thin.
From:
Demon :: d20srd.org
Dretch
Size/Type: Small Outsider (Chaotic, Extraplanar, Evil)
Hit Dice: 2d8+4 (13 hp)
Armor Class: 16 (+1 size, +5 natural), touch 11, flat-footed 16
Challenge Rating: 2
Babau
Size/Type: Medium Outsider (Chaotic, Extraplanar, Evil)
Hit Dice: 7d8+35 (66 hp)
Armor Class: 19 (+1 Dex, +8 natural), touch 11, flat-footed 18
Challenge Rating: 6
Bebilith
Size/Type: Huge Outsider (Chaotic, Extraplanar, Evil)
Hit Dice: 12d8+96 (150 hp)
Armor Class: 22 (-2 size, +1 Dex, +13 natural), touch 9, flat-footed 21
Challenge Rating: 10
Glabrezu
Size/Type: Huge Outsider (Chaotic, Extraplanar, Evil)
Hit Dice: 12d8+120 (174 hp)
Armor Class: 27 (-2 size, +19 natural), touch 8, flat-footed 27
Challenge Rating: 13
Marilith
Size/Type: Large Outsider (Chaotic, Extraplanar, Evil)
Hit Dice: 16d8+144 (216 hp)
Armor Class: 29 (-1 size, +4 Dex, +16 natural), touch 13, flat-footed 25
Challenge Rating: 17
Balor
Size/Type: Large Outsider (Chaotic, Extraplanar, Evil)
Hit Dice: 20d8+200 (290 hp)
Armor Class: 35 (-1 size, +7 Dex, +19 natural), touch 16, flat-footed 28
Challenge Rating: 20
TomB