I am not sure you read the DMG. It doesn't state which average they are discussing.
Uh huh. Where exactly does one find "the average constitution of a monster of a certain role"?
You made this up, right?
What might the average be? The DMG does not state.
It could be:
Str 42 | Con 36, average = 39, +1
Dex 26 | Int 3, average = 14.5, +5
Wis 18 | Cha 7, average = 12.5, +2
resulting in:
AC: Level 30 + 12 + 2 = 44 (MM 43)
Fort: Level 30 + 12 + Str (42-39) +1 = 43 (MM 49)
Reflex: Level 30 + 12 + Dex (26-14.5) +5 + 2 = 49 (MM 38)
Will: Level 30 + 12 + Wis (18-12.5) +2 + 2 = 46 (MM 32)
But this doesn't make any sense. It does the opposite of the intent.
The only thing that makes sense is either:
1) Take the sum of all of the ability scores for the monster, divide by 6 to get the average ability score.
or
2) Take the highest of each defense pair, sum them, divide by 3 to get the average ability score used to calculate a defense.
#2 makes more sense because it determines the current defense on the average defenses for that particular monster, but it looks like they used #1 in some cases in the MM.
From the MM
3. Determine Ability Scores. It’s helpful to think
of ability scores in pairs, each pair corresponding to
one of the three defenses (Fortitude, Reflex, and Will).
Ability scores also help determine the monster’s attack
bonuses, ability and skill checks, and Armor Class.
On average, the highest ability score of a pair is
equal to 13 + one-half the monster’s level. For example,
the target score for an 8th-level monster is 17 (13
+ 4). However, set the ability that governs the monster’s
primary attacks to be 3 higher, or 16 + one-half
the monster’s level. An 8th-level monster that relies on
melee attacks should have a Strength of 20.
6. Calculate Other Defenses. A monster’s level
determines its defenses. A given defense based on an
average ability score is equal to 12 + the monster’s level.
For every 2 points the ability score varies from the average,
adjust the defense by +1 (if higher) or –1 (if lower).
Tarrasque stats:
Str 42 (+31) Dex 26 (+23) Wis 18 (+19)
Con 36 (+28) Int 3 (+11) Cha 7 (+13)
So lets count:
average:
13+1/2 Level = 28
So strength is 14 points higher (instead of just 3)
Dexterity is 2 points lower
Wisdom is 10 points lower
So with guidelines for monsters, the tarrasques defenses should have been 42 before modification:
after modification we have:
Fortitude: excactly 49
Reflex: 41
Will: 37
Seems that the person who made up the tarrasques stat just botched terribly...
edit: made a quick research with the character builder and it seems that they made it a bit easier:
if the highest ability is below average: -1 to defenses
if highest ability is above average: +1 to defenses
and then you can apply aditional penalties or bonuses (-6,-3,0,+3,+6)
your fort :-1, ref -2, will -3 derives from attack stat strength or constitution (which seems to be the brute´s attack stat) +3 higher stat and wisdom or charisma rather low
if you make all tarrasque stats equal you get exactly 42 in all defenses.
So no, i didn´t make it up, i am just able to read.