Last year, Google made a sweeping promise, saying that content stored on smartphones using the next version of its Android operating system, Lollipop, would be automatically encrypted from the moment users power up a device for the first time. The shift, announced shortly after Apple made a similar commitment, rankled law enforcement officials who warned it might make their jobs harder.

But now Ars Technica and others have reported Google isn't actually requiring Android makers and distributors to turn on the encryption feature by default. The instructions given to device makers for using Lollipop say "full-disk" encryption is "optional for Android device implementations without a lock screen." Devices with lock screens must support encryption, but it's only very strongly recommended that the feature be enabled out of the box, not required -- although the document notes that Google expects it to be required in some future version of the software.

Apple, on the other hand, made encryption automatic in iOS 8, which was released in September -- meaning new devices using the system and many older ones were able to upgrade and are protected with the feature. So why is it so much harder for Google to keep its mobile encryption promises?

The answer comes down to how the companies approach the mobile device market. Apple has a vice grip on the iOS devices -- it builds all of the devices that ship with the system, giving it ultimate control over the hardware. That means there's a lot of quality control oversight and the ability to ensure every device is up to handling the technical task of default encryption. It's basically the same approach the company has followed in the personal computer market.

But Android is a much more free-wheeling beast. There are many different manufacturers making devices that rely on the operating system sold by many different carriers around the world. And at it's core, Android is an Open Source project, so there are various forks and offshoots with different security features and update schedules. That results in what experts call fragmentation -- a market where users around the world are using a bunch of different flavors of Android, rather than just the handful of iterations Apple devices might be using.

And fragmentation means Google can have a harder time living up to some technical promises.

A Google spokesperson acknowledged as much in a statement to The Washington Post about the encryption situation: "Due to performance issues on some Android partner devices we are not yet at encryption by default on every new Lollipop device." However, the spokesperson noted that latest version of Google's own Nexus devices are encrypted by default and that users of some previous versions, starting with Jelly Bean, can go into their security settings and enable encryption for the data stored on their devices.