Android thumb is not center_horizontaled at the progress?

Android thumb is not center_horizontaled at the progress?

this is the view,the thumb is not centered at the current progress,i've
seen it's really a problem
http://i.imgur.com/I5DFV.jpg "tooltip"">
and here is xml:
<SeekBar
android:id="@+id/controller_play_progress"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical"
android:maxHeight="25dip"
android:minHeight="25dip"
android:paddingLeft="8dip"
android:paddingRight="8dip"
android:progressDrawable="@drawable/mediacontroller_seekbar"
android:thumb="@drawable/mediacontroller_seekbar_thumb"
android:thumbOffset="8dip" />