There are no rules for how the DM designs NPCs.
The DM has complete freedom over every aspect of world building, from designing geography and cultures, down to the details of any particular NPC. The Dungeon Master’s Guide says:
The world where you set your campaign is one of countless worlds that make up the D&D multiverse, a vast array of planes and worlds where adventures happen. Even if you’re using an established world such as the Forgotten Realms, your campaign takes place in a sort of mirror universe of the official setting where Forgotten Realms novels, game products, and digital games are assumed to take place. The world is yours to change as you see fit and yours to modify as you explore the consequences of the players’ actions.
And this particular change makes plenty of sense. 11 intelligence is smarter than the average human (the commoner stat block has 10 intelligence). Spined devils aren’t dumb.