"We analysed the age, condition, quality and mentality of the player before he arrived, so we knew he was a good player," Benitez told liverpoolfc.tv. "That is why we signed him.
"But to score more than 30 goals in a first season is a fantastic achievement. It's a surprise for everyone, not because we didn't have confidence in him, but because it's his first year in the Premier League."
He added: "I don't know why so many of his league goals have come at Anfield.
"For me, it's a surprise because I'd have thought he'd score more goals away because of his pace and he'd get more space and it'd be easier for him.
"Maybe it's just a coincidence. But the motivation at Anfield is fantastic because of our supporters.
"He's scored at Stamford Bridge and the San Siro in the Champions League, so he has shown he can score anywhere."