Heroes Headcanon
Peter never lost his initial ability.
Peter and Arthur have similar abilities: Peter's empathic mimicry allowed him to use other people's abilities, Arthur can steal other people's abilities to use them himself.
They're different in action, but essentially the same ability, so technically there would be no need for Arthur to take Peter's powers besides to hurt his son.
But, if I remember correctly (it's been awhile since I've watched Heroes) Arthur never seemed to use Peter's ability, the only abilities Arthur was ever seen use, were from people we saw him steal from, but never Peter's. I mean, with Peter's, he should have been able to manipulate electricity because he at least saw Peter do that one, but Arthur never does.
What if, in an attempt to take Peter's powers, Arthur inadvertedly created a mental block, stopping Peter from using his ability. And when Peter took the serum to get his ability back, the mental block only allowed a portion of his ability to be used.
Because, think about it, Peter's ability it largely mentally oriented. Empathy is the ability to feel other people's emotions, and when Peter remembers how people made him feel, he can mimic their ability. It's very emotionally based, which means it's mentally based. A mental block can largely affect a mental ability.
Anyway, that's what I believe.
