That's because what the back button on Android does is partially determined by the developers of the apps you use. If an app is written using Android Framework

That's because what the back button on Android does is partially determined by the developers of the apps you use. If an app is written using Android Framework there's some things that will automatically add an entry to the back-stack, but many actions won't unless the developer explicitly adds them. So naturally, the back button ends up as something of a "I'm feeling lucky" button.