Ubuntu is based on Linux and the questions there are on topic on Unix & Linux. Also there is another Stack Exchange site on Android. But Android is also a Linux as well. So why these three sites? Questions on Ask Ubuntu and Android Enthusiasts are perfectly on topic for Unix & Linux. Shouldn't they be merged with the Unix & Linux Stack Exchange site?
I'll take the Android vs. Linux one specifically:
Unix and Linux Stack Exchange is for topics that can basically be summed up by the first point in their on-topic page: "Using or administering a *nix desktop or server".
Android Enthusiasts Stack Exchange is for topics covering usage of an Android mobile device.
These two categories of devices expose a wildly different interface to their users. They can run very little of the same software. The operating systems you can install on them are different in almost every way except their kernel.
People who are experts in one are not inherently experts in the other - that's why they're different sites.