The PlayStation 4 is selling faster than any Sony console ever, but that success actually has at least one executive worried.

PlayStation 4 has surpassed 10 million sales worldwide after just nine months on the market. That’s a huge number that puts it well ahead of the Xbox One and Wii U, which have reached around 6 million consoles sold each. Despite that big lead, Sony Worldwide Studios president Shuhei Yoshida isn’t sure how to maintain that momentum. A big reason for that fear is because Sony doesn’t really understand why the PS4 is performing so well in the first place.

“I, for one, am a bit nervous because we do not completely understand what’s happening,” Yoshida said in an interview with Eurogamer. “You need to understand why your products are selling well so you can plan for the future, right? [Well, the PlayStation 4] defied the conventional thinking.”

Of course, Yoshida and Sony aren’t clueless. They aren’t blindly stumbling into success like this is some kind of Leslie Nielsen movie. The company has made a lot of smart moves. The PlayStation 4 debuted at an aggressive price, ¬†and Sony made a big effort to speak directly to its biggest fans and potential early adopters.

But the PlayStation 4 is selling so much faster than the PlayStation 3 did. Through its first nine months, the PS3 only sold around 4.3 million units. That’s less than half of what the PS4 is at.

Even the PlayStation 2, Sony’s best-selling system ever, needed around 14 months to get to 10 million.

“As soon as we see a great sales number, our instinct tells us we should be concerned about future sales, right?” Yoshida said. “Are we exhausting all the core gamers?”

It’s possible that Sony is front-loading a lot of its sales. If all of its fans are jumping onto the console now, maybe its momentum will fall off a cliff. That seems unlikely, and Sony is obviously looking into what it can do to prevent that.

“So we want to understand who are these consumers who we do not necessarily consider core gamers,” said Yoshida. “[We need to figure out] who is purchasing the 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.”

PlayStation 4 Sells 10 Million - Gamescom 2014