Ask anyone at Sony, and they're likely to tell you that sales of 10 million PS4s in less than nine months is unvarnished good news. But in an uncharacteristic bit of self-questioning, Sony's head of Worldwide Studios Shuhei Yoshida has expressed some bewilderment and nervousness over the system's quick success.

"It's just beyond our imagination. We are so happy. But I for one am a bit nervous because we do not completely understand what's happening," Yoshida told Eurogamer in a recent interview. "You need to understand why your products are selling well so you can plan for the future, right? It defied the conventional thinking. Lots of people thought the dedicated game hardware might not be needed going forward, but still lots of people are very excited."

While Yoshida said he thinks the lineup of upcoming exclusive games like The Order and Uncharted 4 explains why gamers are excited for the PS4, he said that it still doesn't completely explain the speed at which the system is selling. "I'm asking journalists who ask [about sales] their opinion," Yoshida said. "I'm asking marketing people to tell us why."

Sony's internal reports indicate that many PS4 buyers are people who did not buy a PS3 or who skipped the last generation of consoles entirely, Yoshida said. But that doesn't sufficiently explain why they are getting into the market right now, he said, or whether it's simply a matter of "core gamers" getting into the market earlier than normal.

"As soon as we see a great sales number, our instinct tells us we should be concerned about future sales, right? Are we exhausting all the core gamers?" he said in the interview. "If we sell this number of units, there are no more consumers we can sell to. That's a really terrifying prospect. So we want to understand who are these consumers who we do not necessarily consider core gamers, who are purchasing PS4, and why they are doing it and what they are doing with PS4, so we can create a bit more of a positive future, rather than saying, wow, we have sold to every single core gamer."

While PS4 sales have been quicker than expected, the worry of exhausting the market may be overblown. The PlayStation 3 took just over 13 months to sell its first 10.53 million units worldwide after its release in late 2006. That system then went on to sell more than 75 million units worldwide over its lifetime. It's possible that the universe of "core gamers" interested in a game console has dwindled in the intervening time or moved on to mobile or PC platforms, perhaps. Still, the quick sales of the PS4 thus far could just as easily suggest the opposite has happened.