Why does Grotle evolve earlier than the other Sinnoh starters?

Question

Why does Grotle evolve at level 32, when all the other starters evolve into their last stage at level 34?

Comments

And, on the seventh day, GameFreak said so, and it was good.
TL;DR: GameFreak said so.

Answer 1

There is no definite answer, but the most likely reason is because Torterra has the least base stat total of all Sinnoh starters. Torterra's BST is 525, Infernape's is 534, and Empoleon's is 530.

This is also shown in other scenarios. Venasaur also has a base stat total 525, and similarly to Grotle, Ivysaur evolves at level 32. I suppose GameFreak is deciding to making the weaker starters evolve earlier.

Source: Knowledge and the PokemonDB Pokedex.