One of the things you may have noticed is that your app does not look and work the same on every Android device. Why is that? Well, the answer is both simple and complicated. It's simple in that Android is open source so it can be modified in every way imaginable. It's complicated in that manufacturers do exactly that, there are literally thousands of permutations of Android.
Ok, so that is all well and good, but what does it mean for your app? Well it means that we can't be sure your app will work as expected on every Android device. We try to make sure it's compatible with as many as possible. We do theis by sticking to Google's design directives for Android. Unfortunately many manufacturers depart from those directives to add splashy features to their devices.
For more insight on this topic here is a great article on the fragmentation of Android.