Shadow appears between Button and ImageButton?

Question

Java

(activity_main.xml):

<Button
                style="@style/ButtonsNumber_light"
                android:onClick="onDelAll"
                android:textSize="30sp"
                android:text="@string/b_del_all" />
            <ImageButton
                android:src="@drawable/b_remove_one"
                android:scaleType="fitCenter"
                android:padding="20dp"
                style="@style/ButtonsNumber_light"
                android:onClick="onDel" />
            <Button
                style="@style/ButtonsNumber_light"
                android:onClick="onAddOperator"
                android:text="@string/b_divide" />

file(styles.xml):

<style name="ButtonsNumber_light">
    <item name="android:background">@drawable/button_light</item>
    <item name="android:textColor">@color/colorWhite</item>
    <item name="android:layout_width">match_parent</item>
    <item name="android:layout_height">match_parent</item>
    <item name="android:layout_weight">1</item>
    <item name="android:textSize">32sp</item>
    <item name="android:fontFamily" 
    tools:ignore="NewApi">@font/lato_regular</item>
</style>

file(button_light.xml):

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">

<item android:state_pressed="true" >
    <color android:color="@color/colorGrayLight_pressed" />
</item>
<item android:state_pressed="false" >
    <color android:color="@color/colorGrayLight_not_pressed" />
</item>

</selector>

After that, I get such a layout

. What kind of shadow, stroke because of the ImageButton appears there?