The NPC Caleb Voltiaro in Wake of the Watcher is listed as having Fog Cloud prepared as a second level spell. I'm not sure how this is actually possible for a cleric, since that spell isn't on the cleric list. It's on the list for the Water domain, but he has Slipstream prepared as a domain spell for that level, and he's also using the Oceans subdomain. Is there some alteration for followers of Dagon that I'm not aware of?
Most probable answer:
It's an oversight and not a huge deal.
Your players probably won't notice and (in my opinion) is not worth the headache as it doesn't make him that much more powerful.
If they DO notice you can always say "Yeah...that's weird, isn't it? His demon god seems to change the rules for him a bit" or something along those lines.
In my experience almost every statblock at higher level has one or two minor mistakes, resulting from changes during development or just small mistakes. One AC too little here, one ATK to much there...
The biggest thing coming to mind has to be Auren Vrood, who technically cannot cast his most infamous spell on these boards because his prestige class got changed after his statblock was written down.
It's usually not that important in the grand scheme of things.