According to http://leagueoflegends.wikia.com/wiki/Annie Annie's auto attack has a range of 575, while her Disintegrate has a range of 625, which is higher.
However, many times I seem to have experienced something opposite. I first right click on a minion to AA it; then I Q the minion to last-hit it. And... Having used her AA, when I press the Q button on my keyboard, Annie quite often moves a little forward towards the minion to cast her spell! Just as if her Q's range was LOWER than her AA's range!
Usually this is little of a problem, however, I remember that at least once I failed to last-hit a minion bc of this; Annie was standing right behind a minion of her team, so when I clicked Q she... walked around her minion. This took time and the enemy minion died before she was done with walking.
Video proofs of this:
Normal cast, especially noticeable with the second minion
What am I failing to grasp?