Root LinearLayout不是中心对齐

The Root code is below:

根代码如下:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="https://schemas.android.com/apk/res/android"
xmlns:tools="https://schemas.android.com/tools"
android:id="@+id/satranc"
android:layout_width="fill_parent"
android:layout_height="@dimen/boy"
android:gravity="center_horizontal"
android:orientation="vertical"
tools:context=".MainActivity">
<?xml ver