Over the last few years, iPhone and iPad users have been quick to adopt the latest version of Apple's mobile OS. But iOS 8 seems to be playing out differently. A lot differently.

According to multiple sources, iOS 8 downloads have basically flatlined. Mixpanel reports iOS 8 users currently make up 45 percent of total iOS users, while iOS 7 users still make up 50 percent. Apple's own App Store visit numbers show similar stats: As of October 5th, 47 percent of iOS users are on iOS 8, and 47 percent are on iOS 7. (The last few percent in both analyses are folks on earlier versions of iOS.) With the launches of iOS 6 and iOS 7, the numbers were closer to 60 or 70 percent at this point.

Users could be holding off on updating for a number of reasons, like reported or expected performance issues. For those with older iOS devices, this can be a wise move. As of September 23rd, data from mobile app performance manager Crittercism shows the crash rate on iOS 8 was 64.5 percent higher than on iOS 7.1 (3.29 versus 2 percent). As of October 8th, the iOS 8 crash rate still sits above 3 percent—and that rate skews higher on older devices like the iPhone 5. Historically, new iOS versions can end up hobbling older systems by adding new features, with more bugs. As the device is pushed to its processing limits, performance is diminished. And of course, the botched release of iOS 8.0.1 may have nudged would-be early adopters away from downloading ASAP.

There's also the fact that iOS 8 is a big download, requiring around 6 GB of space. If you only have a 16 GB device, well, you may have to free up some space before you even think about updating.

But slower iOS 8 adoption could also be rooted in the kind of update Apple pushed out this time.

Not for Everyone

"[It's] a nerd release," says developer Andrew Clark of Apple's latest mobile OS. Before you get your Think Different undies in a wad (yeah, those exist), Clark elaborates. "Can you think of any non-power-user features that it’s worth upgrading for? Home Kit and Health Kit are useless until all the accessory makers get on board. Extensions and third party keyboards are great, but a casual user isn’t going to understand or care about them. iCloud Photo Library will be huge but it’s in beta and is switched off by default."

On top of that, another key feature, Continuity, doesn't ship until OS X Yosemite does in October. Continuity lets you hand off tasks and pick them back up right where you left off on a separate iOS or Mac device. And Apple Pay, Apple's NFC-based mobile payment platform, also won't go online until October.

But is the average iOS user aware of those features and their release timeline? A good number don't pay attention to tech news at all, and were seriously confused when U2's new album magically appeared on their iPhone. Perhaps this version's features just aren't as eye catching as the changes Apple made to iOS last year.

What's the difference?

When Apple debuted iOS 7, it was a huge visual departure from its predecessor—one that was debated, hyped, and booed ad nauseam until long after the OS made its public release. People were excited to check it out for themselves. And as app developers updated their offerings to match the new aesthetic, they looked out of place on iOS 6. A number of developers required iOS 7 for their apps so they wouldn't have to support two varying designs. To use your favorite apps, you had to upgrade, so people did.

This isn't true for Apple's latest mobile OS. If you aren't on a new iPhone 6 or 6 Plus, the updates may seem marginal, although they're actually quite significant.

"iOS 8 is laying the groundwork for the next five years of iOS," Clark said. "We’ll look back and see that it was a big turning point, but for now if you haven’t already upgraded, there’s not much pressure to."

That pressure will surely increase with time, particularly as Apple tries to ensure iOS avoids the fragmentation woes that Android faces. "Apple sunsets support on older generation devices," Rob Kwok, Crittercism's CTO and co-founder says. "This forces a regular upgrade cycle that reduces fragmentation." Kwok expects iOS 8 adoption will hit the 60 to 70 percent level within the next 30 to 45 days.

So perhaps users are just getting savvier and playing it safe when it comes to app updates. Or perhaps they're just waiting to upgrade to iOS 8 when it's got the features they need. Regardless, even with slower adoption, iOS 8 has already proven it's not the next Windows 8.