3个button的weight都是1，为什么第一个始终要宽一些呢？

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >    <EditText
        android:id="@+id/editPromot1"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:ems="13"
        android:enabled="false"
        android:focusableInTouchMode="false"
        android:inputType="number" />    <EditText
        android:id="@+id/editPromot2"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:ems="13"
        android:enabled="false"
        android:focusableInTouchMode="false"
        android:inputType="" />
    
    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_weight="1" >        <EditText
            android:id="@+id/editNumber"
            android:layout_width="wrap_content"
            android:layout_height="fill_parent"
            android:layout_weight="4"
            android:ems="13"
            android:enabled="false"
            android:focusableInTouchMode="false" >            <requestFocus />
        </EditText>        <ImageButton
            android:id="@+id/imageDelete"
            android:layout_width="wrap_content"
            android:layout_height="fill_parent"
            android:layout_weight="1"
            android:src="@drawable/sym_keyboard_delete" />    </LinearLayout>    <TableLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_weight="5" >        <TableRow
            android:id="@+id/tableRow1"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:padding="3dp" >            <Button
                android:id="@+id/btn1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="1" />            <Button
                android:id="@+id/btn2"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="2" />            <Button
                android:id="@+id/btn3"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="3" />        </TableRow>        <TableRow
            android:id="@+id/tableRow2"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:padding="3dp" >            <Button
                android:id="@+id/btn4"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="4" />            <Button
                android:id="@+id/btn5"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="5" />            <Button
                android:id="@+id/btn6"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="6" />        </TableRow>        <TableRow
            android:id="@+id/tableRow3"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:padding="3dp" >            <Button
                android:id="@+id/btn7"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="7" />            <Button
                android:id="@+id/btn8"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="8" />            <Button
                android:id="@+id/btn9"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="9" />        </TableRow>        <TableRow
            android:id="@+id/tableRow4"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:padding="3dp" >            <Button
                android:id="@+id/btnx"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="*" />            <Button
                android:id="@+id/btn0"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="0" />            <Button
                android:id="@+id/btnj"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="#" />        </TableRow>        <TableRow
            android:id="@+id/tableRow5"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" >            <ImageButton
                android:id="@+id/imageButton1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:src="@drawable/sym_action_call" />        </TableRow>    </TableLayout>

</LinearLayout>TableLayout是4行，每行3个按钮，3个按钮的weight都是1，但是第1列按钮始终要宽一些，这是怎么回事呢？