There's actually a
dedicated thread for the damage calc, but to answer your questions it's just due to the
Gen 4 damage formula, which rounds down after every step, including between each type effectiveness multiplier. So Ice moves sometimes do less damage to Kingdra than Electric moves of equal power because Water halves the damage and rounds down before Dragon doubles it.
Since you didn't actually say which Pokemon you were attacking with Crunch/Superpower I'm assuming a similar situation. Either way, everything is explained by rounding.