It would make sense, given the comparative power level of Zariel we're seeing here. She and Demogorgon have the same CR, but he's arguably the strongest Demon Prince while she's likely one of the weaker level-ruling Archdevils.
And it could easily be explained in gaming terms - the versions of the Demon Princes seen in OotA could just be avatars, or similarly weaker versions, as the result of the botched summoning in that adventure. The Demon Princes' true forms would likely be somewhat stronger.
+8 proficiency skill? Daaaamn. I thought +6 was the cap!
My lvl 6 party can beat your party of lvl 12 bitter naysayers!Can't wait for the bitter nay sayers to get here. They'll say that Zariel is weak and that their lvl 12 party can easily beat her.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.