Admob大小不会填充父级

我将发布下面的代码,我的问题是这个,在模拟器(Genymotion Galaxy Nexus 4.2.2)中,admob横幅拉伸并填充其父级:

在真实设备(Galaxy Nexus 4.3)中,admob横幅不会填充其父级的整个宽度,在两侧留下两条白色条纹:

这是布局:

这是活动的onCreate:

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);    
        requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);    
        setContentView(R.layout.activity_detail);


        ActionBar actionBar = getSupportActionBar();            
        actionBar.setDisplayHomeAsUpEnabled(true);

        WebView myWebView = (WebView) findViewById(R.id.webview);    
        WebSettings webSettings = myWebView.getSettings();
        webSettings.setJavaScriptEnabled(true);    
        Bundle b = getIntent().getExtras();
        url = b.getString("url");           
        myWebView.loadUrl(url);
        myWebView.setWebViewClient(new MyWebViewClient());


        // Initiate a generic request.
        AdRequest adRequest = new AdRequest.Builder().build();    
        adView = (AdView) findViewById(R.id.adView);
        adView.loadAd(adRequest);
    }

我究竟做错了什么？我已经检查了很多,但我无法弄明白......