AutoCAD Civil 3D :: Quantity Takeoff Calibration Variances

For a highway project rail overpass there are 30' curved retaining walls - the height changes continuously and the footing and wall thickness steps according to the CalTrans Type-1 wall stepping standard. So the subassembly is a custom object. I'd like to query the C3D community knowledge for the QTO accuracy - and find an answer to these two questions.
 
The retaining wall area QTO by corridor payitem code is 3,000 m2 and by volume report (sample line) is 5,500 m2; why is there a difference?The volume given by volume report (6,300 m3) is 40% more then the volume found by CalTrans and the contractor with a 2D digitizer QTO (4,000 m3) - who is correct C3D or CalTrans?

I am working on the QTO right now and I'd like to move to the schedule this week.