Strange Surface Lines

[kappasigma]

When I am viewing a surface (either volume or terrain) in 3D orbit, the surface is shown with perpendicular lines connecting the boundary of the surface to the datum. In the tutorials, the surfaces do not have these additional lines. My hand calculation of the excavation is about 30% smaller (500 CY) than AutoCAD LD's calculation, so I'm wondering if rather than calculating the difference between my OG surface and my FG surface, AutoCAD is in fact calculating the difference between the FG surface and the datum.

I checked my stratum (correct), surface cross-sections (look accurate), and breaklines (elevation range is correct). I'm all out of ideas, and no one else seems to be having this problem. I've attached a pic of the surface in 3D orbit. 

My questions are as follows:

1. Are these merely reference lines to show how the surface connects to my plan drawings, or is it possible AutoCAD is taking a volume surface all the way to my datum?

2. How do I remove these lines / make sure AutoCAD is calculating volumes correctly?

[kappasigma]

So after some experimentation, it's looking like the problem was that I didn't create non-destructive breaklines at the boundaries. Once I started trimming the boundary edges, my hand calcs matched my cut-fill volumes in AutoCAD.