Microsoft's Xbox One is racing ahead of Sony's PlayStation 4

Microsoft's Xbox One is on a roll. 

September was the third month in a row that the Xbox One outsold its competition, the PlayStation 4.

That's a big change: the PlayStation 4 is far and away the most popular game console in the world. Over 40 million PlayStation 4 consoles are out in the wild; Sony took enough of a lead with the PS4 over the Xbox One that Microsoft stopped reporting sales numbers, instead choosing to report "hours played," and other non-traditional metrics.

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But things have changed.

Ever since July 2016, Microsoft's Xbox One has beaten the PS4 in month-to-month sales. Keep in mind that the Xbox One and PlayStation 4 have been on sale since the 2013 holiday season - for the vast majority of that time, PlayStation 4 has been on top. There were a few months here and there where the Xbox One beat the PS4, but Sony's lead has held strong for years now.

So, what's going on? A few factors play into the change. 

First and foremost, there are a ton of PlayStation 4 consoles already out in the wild.

AP Photo / Nam Y. Huh

It stands to reason that demand for the PS4 has cooled over time as the group of (millions of) people have their demand met by simply buying a console. Again, there are over 40 million PlayStation 4 consoles in homes.

The second major factor is exclusives.

Microsoft / Reddit

"Forza Horizon 3" is a crazy good, crazy gorgeous racing game on the Xbox One and Windows 10 platforms.

Microsoft's Xbox One has two major blockbuster exclusive games this year in "Forza Horizon 3" and "Gears of War 4." Not only are both games exclusive, but they're critically-acclaimed entries in beloved franchises. And these are franchises you can only play on Microsoft's platforms, Xbox One and Windows 10.

And the third major factor, which applies mostly to September sales, is the Xbox One S.

Florence Fu / Business Insider

The Xbox One S.

Microsoft's got a new version of the Xbox One that's slimmer, prettier, and the same low price as the original Xbox One ($299). In fact, the Xbox One S was the highest-selling console of any gaming hardware in September. That's an especially interesting fact when you consider that Sony released a slimmer version of the PlayStation 4 in September as well - Microsoft announced the Xbox One S months earlier in June, and was able to cash-in on a summer of marketing and pre-orders leading up to its launch in late August.

But don't count Sony's PlayStation division out just yet. We're talking about the most profitable part of Sony, after all. And this holiday season is full of huge releases from the company.

Between the launch of the slimmer PlayStation 4 in September, the launch of PlayStation VR in October, and the launch of the more powerful PlayStation 4 Pro in November, Sony's got a lot of potential to make up for the past few months. Whether consumers will make that a reality is another question altogether.