Well, 1st of all, if you'll notice, both evolved forms have sheldder on different parts of their bodies, so that explains THAT.
and 2: If there is more than 1 evolution, they try and fit the conditions to the kind of player that you are. If you want to evolve a pokemon, you hve to know how. for example, if you're the kind of person who would PREFER a male psychic -type over a female one, or you prefer a physical attacker, go with gallade. If you prefer females, don't have a gen V or IV game, or Prefer special attackers, go with Gardevoir.