自定义seekbar样式后thumb 在进度条两端时不是中心停在端点，向两边偏了点，求解呀！

seek_style.xml<?xml version="1.0" encoding="utf-8"?>
<layer-list 
    xmlns:android="http://schemas.android.com/apk/res/android" >
    <!-- 背景图 -->
    <item android:id="@+android:id/background" 
        android:drawable="@drawable/seekbar_dn" 
        />
    <!-- 第二进度图 -->
    <item android:id="@+android:id/SecondaryProgress" 
        android:drawable="@drawable/seekbar_dn" />
    <!-- 进度 -->
    <item android:id="@+android:id/progress">
        <clip>  
            <nine-patch android:src="@drawable/seekbar" />  
        </clip>
   </item> 
</layer-list>thumb.xml<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <!-- 按下状态 -->
    <item android:state_pressed="true"
        android:drawable="@drawable/point" />    <!-- 焦点状态 -->
    <item android:state_focused="true"
        android:drawable="@drawable/point" />
    
    <!-- 默认状态 -->
    <item android:drawable="@drawable/point" />  
    
</selector> 
<SeekBar
    android:id="@+id/seekBar"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginTop="27dp"
    android:layout_toLeftOf="@+id/brightness1"
    android:layout_toRightOf="@+id/brightness2"
    android:max="205"
    android:maxHeight="6px"
    android:minHeight="6px"
    android:progressDrawable="@drawable/seekbar_style"
    android:thumb="@drawable/thumb"
    />这是图的问题还是方法的问题？