XML : The preview in Android Studio is not the same as the app running on the emulator

I'm new to Android programming and I'm already facing a problem with the simplest thing one could imagine: placing a button on the screen. In the preview of the activity.xml file everything is fine but in the emulator it's messed up, the buttons and labels are not where they should be.

Here is an example: I put a button to the left of the floating action button but in the emulator it's way above it.

Android Studio Preview

Emulator

Here is the XML code:

  activity.xml        <?xml version="1.0" encoding="utf-8"?>  <android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"      xmlns:app="http://schemas.android.com/apk/res-auto"      xmlns:tools="http://schemas.android.com/tools"      android:layout_width="match_parent"      android:layout_height="match_parent"      android:fitsSystemWindows="true"      tools:context="com.example.ed.wtf.MainActivity">        <android.support.design.widget.AppBarLayout          android:layout_width="match_parent"          android:layout_height="wrap_content"          android:theme="@style/AppTheme.AppBarOverlay">            <android.support.v7.widget.Toolbar              android:id="@+id/toolbar"              android:layout_width="match_parent"              android:layout_height="?attr/actionBarSize"              android:background="?attr/colorPrimary"              app:popupTheme="@style/AppTheme.PopupOverlay" />        </android.support.design.widget.AppBarLayout>        <include layout="@layout/content_main" />        <android.support.design.widget.FloatingActionButton          android:id="@+id/fab"          android:layout_width="wrap_content"          android:layout_height="wrap_content"          android:layout_gravity="bottom|end"          android:layout_margin="@dimen/fab_margin"          android:src="@android:drawable/ic_dialog_email" />    </android.support.design.widget.CoordinatorLayout>        content.xml            <?xml version="1.0" encoding="utf-8"?>      <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"          xmlns:app="http://schemas.android.com/apk/res-auto"          xmlns:tools="http://schemas.android.com/tools"          android:layout_width="match_parent"          android:layout_height="match_parent"          android:paddingBottom="@dimen/activity_vertical_margin"          android:paddingLeft="@dimen/activity_horizontal_margin"          android:paddingRight="@dimen/activity_horizontal_margin"          android:paddingTop="@dimen/activity_vertical_margin"          app:layout_behavior="@string/appbar_scrolling_view_behavior"          tools:context="com.example.ed.wtf.MainActivity"          tools:showIn="@layout/activity_main">            <Button              android:layout_width="wrap_content"              android:layout_height="wrap_content"              android:text="New Button"              android:id="@+id/button"              android:layout_marginBottom="65dp"              android:layout_alignParentBottom="true"              android:layout_centerHorizontal="true" />      </RelativeLayout>