When creating Android apps, your final product isn't necessarily predictable across the hundreds of different Android models. Your app can run into hardware issues like memory size, and sometimes you'll find that certain hardware API's that only work on newer devices. Also, some devices have different UI paradigms, while others add their own UI layer on top of the app. This means on some devices your app may have a complete failure to install, it may crash, or the resolution may give your app ridiculous layouts.
One way to ensure consistency is to test out all of your Android apps yourself, requiring plenty of phones and man-hours to install and run the tests manually. Or you could use Helsinki-based Bitbar's suite of Testdroid Android testing applications to automate and quickly run these tests across over 100 devices.