I think it's because the quirk has evolved. I'm basing this on the statement from the original user that the quirk is beyond the point of singularity. Just a thought not sure though. So new abilities are coming forth like him being able to speak to his predecessors while all might could only see them when he had one for all.
Yeah that is what I'm thinking as well. It's like an egg that the 8 previous users was incubating. So now it is hatching and Deku now needs to learn to use it and mature a long with it. I also believe that this is also the reason he couldn't control it at all at first. So I'm not sure if this is the reason it hurt him so much..was it his body or ofa?
Prob both, his predecessors were all pretty muscular...I wonder if they all got ofa in their early teens.
He's got to master the quirks of his predecessors (excluding AllMight who was also born quirkless)...why wasn't All Might able to channel them too I wonder.