Por que Tim não reconheceu essa voz?

Sim, é uma questão de Tim e seu pai serem separados. O diretor Rob Letterman falou sobre isso em uma entrevista concedida a ETonline:

ET: And you go through Tim's trauma of his father leaving and how young he was because, otherwise, you would think he would recognize his father's voice.

Rob Letterman: We disguise it. We talked about it endlessly, about, "Why doesn't he recognize his dad's voice?" There were debates. Justice and I were talking about it forever. And Ryan, like, it was a real discussion point. [Laughs] Where I ended up in my mind was it's not the voice that he's hearing, it's the connection to his father. The reason he doesn't recognize the voice, it's not the literal voice. It's that Tim is estranged from his dad and through this experience, he's getting closer and closer. That's why this voice in his head is purely an emotional connection to Pikachu. That's sort of my take on it.

_{'Detective Pikachu': The Director Answers All of Our Question About That Big Twist, John Boone for ET, May 13, 2019}

Embora o diretor do filme pareça ter a impressão de que A voz de Pikachu é diferente da voz de Harry, a novelização (baseada no script) indica que Pikachu e Harry falam com a mesma voz exata. Isso pelo menos confirma que a voz que Tim não estava reconhecendo era a própria voz do pai.

Para o registro, a voz o lembra de um detetive de TV, sugerindo que ele se afastou suficientemente de seu pai para simplesmente não se lembrar de como é sua voz.

"I know you can't understand me," the Pikachu said slowly in a surprisingly deep voice, kind of like the detective from the movie that had been playing. "But put down the stapler, or I will electrocute you."
"Did you just talk?!" Tim exclaimed.
He was so shocked, he dropped the stapler.

Detective Pikachu couldn't believe it, either. "Whoa! Did you just understand me? Wait—wait, wait, wait, wait. That is heavy eye contact right there. You heard me." "No, no, no. No, no, no. Stop, stop, stop, stop!" Tim shook his head, refusing to believe the impossible—that he could actually understand a Pokemon. "Yes, you did! This is amazing," yelled Pikachu, excitedly. "You can understand me, I’ve been so lonely!”