[android-developers] why the underline is shorter than text's length --spinner android

Hi all:
I make a spinner on android 4.1.2, and the resolution is hdpi. The default style of spinner is the attached picture. but my underline(the red line, please see the picture) is shorter than the text's length. Why?
my code ：
 <Spinner

            android:id="@+id/spinner_filter"

            android:layout_width="wrap_content"

            android:layout_height="wrap_content"

            android:layout_marginLeft="12dp"

            android:layout_alignParentLeft="true"

            />
Thanks!
Hilda

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