To add on and Clarify Blaziken's answer, with that amount of HP, paired with not-so-good defensive power, he will most likely survive almost any hit, and with this amount of HP that would deal 583*2 if the Sash will activate, thus getting to damage of 1166, and no pokemon has enough HP to take this kind of damage, not even Blissey. His pre-evolution, acts the same way, so that's why it is Uber.