From what I can tell from what I've heard online and offline - and bear in mind it's subjective territory:
1. Goku has had special methods of training, whereas we don't know what means 17 had of training, and have no reason to assume that they're special.
2. Android 17 never really shone out in Z; he came across as a minor villain, so it's weird that he's on the top tier now.
3. The Freeza powerup is more believable because he probably has more resources available, and even then, not everyone finds that believable either.