If spell DCs are supposed to be 10 higher than a creatures spell attack bonus (Jason Bulmahn even says as much during the 4th episode of Knights of Everflame around the 38 minute mark), there seems to be quite a few monsters/npcs that don't follow that rule and I'm wondering which (if any) of these numbers are right.
From Bestiary:
Astral Deva (DC 36, attack +28)
Brain Collector (DC 26, attack +18)
Young Blue Dragon spellcaster (DC 28, attack +21)
Adult Blue Dragon spellcaster (DC 34, attack +27)
Ancient Blue Dragon spellcaster (DC 41, attack +35)
Young Green Dragon spellcaster (DC 26, attack +20)
Adult Green Dragon spellcaster (DC 33, attack +26)
Ancient Green Dragon spellcaster (DC 39, attack +33)
Young Red Dragon spellcaster (DC 29, attack +23)
Adult Red Dragon spellcaster (DC 35, attack +29)
Ancient Red Dragon spellcaster (DC 42, attack +36)
I think you get the idea with the dragons, all of them are generally between 8 and 6 points off, with ancient golds only being a 3 point difference between attack and DC
Water Mephit (DC 17, attack +9)
Faerie Dragon (DC 18, attack +10)
Drider (DC 24, attack +17)
Ghost Mage (DC 29, attack +23)
Gnoll Cultist (DC 22, attack +10)
Green Hag (DC 20, attack +14)
Merfolk Wavecaller (DC 18, attack +10)
Dark Naga (DC 26, attack +18)
Guardian Naga (DC 29, attack +21)
Morrigna (DC 35, attack +30)
Dandasuka (DC 22, attack +15)
Treerazer (DC 49, attack +43)
Zaramuun (DC 37, attack +31)
From Age of Ashes
Voz (DC 23, attack +15)
Anadi Elder (DC 25, attack +17)
Belmazog (DC 27, attack +19
Mialari Docur (DC 29, attack +23)
Generally it looks like it's only a 2 point difference, but there's the occasional 4 (and oddly enough 5) point difference. And then of course there's ancient gold dragons, with their 41 DC and +38 to hit.
If you look it scales with level. For creatures that are just spell casters it's always 10, but for creatures who are casters and fighters the difference decreases as level increases.
I think it was too help their cantrips, because there aren't many spells with attack rolls.
But I don't like it at all, what it does is make them a lot better at counteracting than being dispelled, which doesn't make much sense.
If you look it scales with level. For creatures that are just spell casters it's always 10, but for creatures who are casters and fighters the difference decreases as level increases.
Doesn't really seem to matter if they are more caster or fighter focused. Voz, for example, in the Age of Ashes AP, is a wizard. She's not built to be swinging around her dagger. Nagas, while decent in melee, have always been caster oriented creatures. The merfolk wavecaller is specifically called out as being a caster. The ghost mage, well, it's in the name.
Ultimately I don't care about the difference in numbers, as long as it's what the numbers are supposed to be. If the difference is intended, maybe we'll get more insight into it once the creature building rules/GMG comes out.
The only way I can think of to counteract this tragedy is to personally craft every custom creature to have as less glaring errors as possible, however taxing such task may be. Which I definitely plan to do so, as having different standards alone is like claiming there are two gravitational constants in our world (which is blatantly false, by the way). Anyway, IMHO, all above.