Center TextView horizontally in RelativeLayout

I'm trying to center a TextView vertically in a RelativeLayout. The preview of Android Studio displays the layout how I want it:

But my phone shows this (I use a custom font):

Why is the TextView not centered on my device?

The XML:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://ift.tt/nIICcg"
    android:layout_width="match_parent"
    android:layout_height="110dp"
    android:padding="10dp"
    android:background="@drawable/favourites_item_bg" >

    <RelativeLayout
        android:id="@+id/color"
        android:layout_width="90dp"
        android:layout_height="90dp"
        android:background="#63FF87" />

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_toRightOf="@+id/color"
        android:layout_toEndOf="@+id/color"
        android:padding="10dp">

        <TextView
            android:id="@+id/num"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="1."
            android:textStyle="bold"
            android:textSize="18sp"
            android:textColor="#000000"
            android:layout_alignParentTop="true"
            android:layout_alignParentLeft="true"
            android:layout_alignParentStart="true" />

        <TextView
            android:id="@+id/hex"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="#63FF87"
            android:textColor="#000000"
            android:layout_centerVertical="true"
            android:layout_alignParentLeft="true"
            android:layout_alignParentStart="true" />

    </RelativeLayout>

</RelativeLayout>