The rule behind Android app structure

Question

I'm trying to understand the rationale behind the app structure in Android's OS.

Some native apps like People and Clock completely remove the top action bar and represent a page organization with tabs instead. In these particular apps, the tabs are icons.

While I think this is perfectly fine because including an action bar with tabs can redundantly display the title in two places, but navigation is relied on the historical back and they seem more modal like iPhone, which I also think is fine since users frequently use this back button action.

Is there a larger rule I'm missing on why Android mixes the use of tab only navigation vs action bar and tabs together? I just would like a more experienced viewpoint other than "they just do it because you can."

Answer 1

Every app inside the Android pile of code, is replaceable by manufacturers of devices implementing Android OS. Every app is also replaceable by the users of Android OS. The same goes for development of Android Apps, it's free to implement any design of Interaction you want since there are no Gate Keepers nor Key Masters restricting developers from uploading anything they want. There are design rules, but no one but the users are checking its validity.

So there are rules (a.k.a Design Principles), but they are not always honored by developers of Android Apps.