Android XML attribute style [closed]

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Why is it that some attributes like android:layout_width is put together with an underscore? While e.g android:textColor is instead using camel-casing?

Why not stick entirely with one style (either underscore or camel-casing)?

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These are not cases of different style. The second case is a simple attribute which is used by an element. The first one is an attribute which is assigned to an element, but is used by a layout. This is emphasized by the underscore-separated prefix. If you happen to look at such an attribute as android:layout_marginTop, you'll see that these two seemingly separate styles are actually parts of a single idea.