Wow, this is news to me as well. Good thing I have well water and don't actually even need the dechlorinator (although I use it anyway, just in case).
Why the discrepancy? If you Python out 30% and then redose for the FULL capasity of the tank, why would you not do the same if using the bucket method (as 247plants stated)?
The reason you wouldnt do the same is because when you dose the dechlor in the bucket, then pour it in the tank you are putting dechlored water into a tank full of already dechlored water.
If you are just dumping tap water in the tank and then adding dechlor, then yes you should dose for the tank.