解释下，为什么LinearLayout在FrameLayout下面就会固定到底部，linearlayout底部

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <FrameLayout
        android:id="@+id/content"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1" >
    </FrameLayout>

<LinearLayout
        android:layout_width="match_parent"
        android:layout_height="70dp"
        android:background="?attr/colorPrimary" >

        <TextView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:text="abc"/>
</LinearLayout>