While this is a good explanation for why Fort saves are higher than Will saves on published monsters, I don't consider it a reasonable justification. There's no reason monsters should have ability scores modifying their defenses. Just make the non-AC defenses average 12+level, with some higher and some lower as seems appropriate.
This is a semi-reasonable thing to do. But, it is not the guideline.
If people actually follow the guideline, even when crafting their own monsters, there will be a propensity in most home brewed monsters to have higher Forts. It's the way the game is designed.
Granted, the Monster Builder does not follow the guideline. It has it's own new rules.
Creating a Tarrasque with it results in:
AC:42 Fort:43 Reflex: 42 Will: 41
In fact, every role ends up with these same scores except for AC. There, Artillery and Brute have AC 42, Soldier has AC 46 and everything else has AC 44. Boring.
But by default, Fort is higher with the Monster Builder, just like it is for published monsters.
And their new rule here appears to be (except for 3 roles) Fort = AC -1, Reflex = AC -2, and Will = AC -3 (which is a simplification and similar to Fort = AC -1.39, Reflex = AC -2.75, and Will = AC -3.31 average of the MM).
Ability scores modifying monster defenses is why there are monsters like the Tarrasque with 32 Will and 49 Fortitude. Not fun!
The difference between Str and Wis for a Tarrasque is 12, not 17. This is just an error. The defenses for a Tarrasque should have been:
Solo: Boost AC, Reflex, and Will by 2.
AC: Level 30 + 12 + 2 = 44 (MM 43)
Fort: Level 30 + 12 + Str (42-22) +10 = 52 (MM 49)
Reflex: Level 30 + 12 + Dex (26-22) +2 + 2 = 46 (MM 38)
Will: Level 30 + 12 + Wis (18-22) -2 + 2 = 42 (MM 32)
The difference should have been 10, not 17. Still a big chunk of a D20, but not quite as bad as it was printed. And I would have a lot less of a problem with a 52 Fort than with a 32 Will. Throw poison at this engine of death all day long and it does nothing.
Note: The DMG guideline should have been the average of the best Str/Con, Dex/Int, and Wis/Cha (i.e. the average scores used to determine defenses, not the overall average of all scores, those other scores are irrelevant to defense). Otherwise, the numbers climb too much for low stat extremes like a Tarrasque. Using this interpretation, the Tarrasque would have been AC: 44, Fort: 49, Reflex: 43 and Will: 39.
Look at the Godforged Colossus:
Elite: Boost AC, Fort (best defense for Elites), and Reflex by 2.
AC: Level 29 + 12 + 2 = 43 (MM 43)
Fort: Level 29 + 12 + Str (40-25) +7 + 2 = 47 (MM 48)
Reflex: Level 29 + 12 + Dex (21-25) -2 + 2 = 41 (MM 40)
Will: Level 29 + 12 + Wis (34-25) +4 = 45 (MM 45)
The numbers are reasonable using the DMG guideline (AC: 44, Fort: 44, Reflex: 40 and Will: 42 with what the DMG should have been). The numbers are less reasonable for a Tarrasque since it has so many really low stats to bring down the average.
But, I do think that a huge stupid brute Tarrasque should be a lot more susceptible to Will attacks than Fort Attacks. Not necessarily by 17, but easily by 8 or so.
And I think the default AC:44 Fort:43 Reflex: 42 Will: 41 from the Monster Builder is just plain boring and too similar, shy of DM intervention. Yawn. Faster monsters should have higher Reflex, tougher monsters should have higher Fort, and higher strength of will monsters should have higher Will. And the DM shouldn't have to tweak this in the Monster Builder to get a reasonable result. IMO.