Admob size does not fill parent

The answer is on AdMob website (about smart banner size):

When an image ad won't take up the entire allotted space for the banner, we'll center the image and use a hexagonal textile filler (see image) to fill up the remaining space. Note that AdSense backfill ads will be centered and have "transparent" filler.

The second image has an AdSense ad and hence the filler is transparent.

I was having same issue and then I change adSize to ads:adSize="SMART_BANNER" so this is how my AdView block look like in xml file.

<com.google.android.gms.ads.AdView
        android:id="@+id/adView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_alignParentBottom="true"
        ads:adSize="SMART_BANNER"
        ads:adUnitId="@string/banner_ad_unit_id">
    </com.google.android.gms.ads.AdView>