To answer your question, one of two things is possible.

- The base stats for the three Kalos starters were calculated incorrectly.
- Delphox has better stats than the other two Kalos starters.

The second possibility is the *much* more likely one, because unfortunately, the starters are **not** equal in every generation. Take into account, for instance, Charizard, Blastoise, and Venusaur. Charizard's base stat total is 534, Blastoise's is 530, and Venusaur's is 525.

This is mentioned in-game as well. For instance, when you are about to choose your starter in some Pokémon games, it will say something like "This is Charmander. He'll be tough to raise." or "This is Squirtle. Raising him will be worth it." or "This is Bulbasaur. He'll be easy to raise." (I didn't get the wording exactly right, but I think you got the gist.)

This implies that (1) evolutions occur earlier for the Grass starters (e.g. Ivysaur evolves at level 32, while the other two evolve at 36) and (2) that the beginning base stats are higher for the Grass-type starter and lower for the Fire-type (but end up inverting once they reach their final forms).

So yes, Delphox's stats are higher, but if you look at Fennekin's base stats, you'll see that it has the lowest of the three Kalos starters.