There's no point talking about this, because their power levels change drastically based on how strong the plot needs them to be, because the writer is awful at writing. Kenpachi, for example, had to use a two hand grip to cut axe-espada at all, but now he's slicing through 0, who's supposed to have iron skin as one of his main abilities, as if he's made of butter.
Do we even know if he even really is as powerful as he claims to be? He's getting sliced up much more easily than the other espada as you said. Is he just erasing his "1" because he thinks he is that powerful and none of the others are around to kill him over it?
Either way, it's kind of a **** plot element