Grey area on nexus 9

I'm facing an issue on nexus 9.

I have a "basic layout" for my view:

<RelativeLayout android:id="@+id/micContainer"  android:layout_width="match_parent" android:layout_height="wrap_content" 
        android:background="#99ffffff" android:layout_alignParentBottom="true" android:layout_marginBottom="15dp" >

        <LinearLayout   android:layout_width="wrap_content" android:layout_height="wrap_content" android:orientation="horizontal"  android:layout_centerInParent="true">
            <TextView android:id="@+id/main_ment_0" style="@style/ment_txt" android:text="" android:layout_margin="5dp"   />            
            <TextView android:id="@+id/main_ment_1" style="@style/ment_txt" android:text="" android:layout_margin="10dp" android:layout_marginLeft="10dp" android:visibility="gone" />          
            <TextView android:id="@+id/main_ment_2" style="@style/ment_txt" android:text="" android:layout_margin="10dp" android:layout_marginLeft="10dp" android:visibility="gone" />
        </LinearLayout> 

        <ImageView android:id="@+id/main_mic" android:layout_width="70dp" android:layout_height="70dp" android:background="@drawable/mic_0" android:layout_margin="3dp" android:layout_centerVertical="true" />

</RelativeLayout>  

This works well on a nexus 5:

However on a nexus 9, a grey area is visible and I can't figure why:

Do you have any idea why this grey area is visible (and if yes why it doesn't show on nexus 5) ?

Thank you in advance for your help.