According to Bulbapedia, when you uncover a (!!) bush on the Pokewalker, you can find either a Group B Pokemon or a Group A Pokemon for that route, while a (!!!) bush always gives you a Group A Pokemon.
For example, on Yellow Forest a Group B Pokemon are Pikachu with Flail and Pikachu with Volt Tackle, while the Group A Pokemon are Pikachu with Surf and Pikachu with Fly.
However, I've caught about six Pikachu now from (!!) bushes and they're all Group B (Pikachu with Volt Tackle). I haven't seen a single (!!!) bush, either.
Can a (!!) bush really produce a Group A Pokemon?