What is the white line near the horizon within Synthetic Vision while using a Stratus 2?

While using a Stratus 2 with Synthetic Vision, you will see a white line near the horizon.  This white line represent a flat horizon relative to your altitude.  As you climb higher, the white line will appear to sit higher above the terrain.   

Image 1.  White line below mountain peaks at lower altitude.  

 

Image 2.  White line above terrain at high altitude.