When it comes to smartphone operating systems, there are really only two games in town—Google's Android and Apple's iOS.

Together, these two systems run on more than 95% of the world's smartphones, according to data from Newzoo.

However, the fortunes of the two systems are wildly different depending on what geographical region is examined.

Newzoo reported that Android's smartphone share in the US was 48.3% in November 2017, up slightly from 45.7% in October 2016.

But Android remains the dominant OS on a global scale, used by 75.9% of smartphones worldwide in November 2017, according to Newzoo.

Android's larger global share is partially linked to its popularity in emerging economies, where price remains an important factor in consumers' smartphone purchase decisions.

That's reflected in figures for smartphone shipments worldwide from International Data Corporation (IDC). The firm estimates Android smartphones made up 85.1% of all smartphones shipped globally in 2017, compared with Apple's 14.8% share.

IDC doesn't foresee much change over the coming years, with that ratio projected to remain nearly the same in 2021.

Google's decision to make Android open source and available to almost any smartphone manufacturer has made it pretty much the default operating system on any smartphone—save those built by Apple.

As the ranks of smartphone users in developing economies, such as India and Indonesia, continue to swell, it's most likely that consumers new to the devices will continue to purchase Android phones. That outcome will be driven by the iPhone's premium pricing strategy and ongoing perception as a luxury product.

But just because Android remains the more popular OS globally doesn't mean that it is without challenges. The Android device ecosystem remains badly fractured.

According to figures from Android, less than 1% of Android devices were running its most recent software version, called "Oreo," as of early January. The majority of Android devices were either running the "Marshmallow" or "Nougat" versions—the second and third oldest software versions, respectively.

That kind of fracturing can create challenges for app developers that are considering how to best prioritize their Android efforts.