I think Hugh was supposed to be one of the Borg babies, as in he was meant to be one that was never an individual to begin with. It's why he turned more easily and developed emotions and stuff better than Seven. With her she DID have a life before the Borg and thus they would have inhibited her ability to feel emotions more to ensure control.
That's a good explanation. It would explain why Hugh was more child-like after he was cut off from the Borg. Seven would have both her Borg personality and Annika personality fighting against each other. Plus the fact that the Annika personality was severely violated in childhood would make her kind of a head case.