I've been wondering how a Composite volume calculation, which does not use the end areas can be generated by a Sectional Materials list. While browsing through the help files I found some info on how the calculations are performed which might help you.
The other thing that I would try is to print out a volume report for each of the three calculation methods, then start looking section by section to see if you can figure out where the extra volume appears along your alignment. Keep in mind that the end area and prismoidal calculations are based on interpolating between the sections, but the composite calc uses ALL of the surface data within the limits of the sample lines. If your EG surface has a spike hidden somewhere in the middle then you could very well be getting a lot of extra volume that wouldn't show up in the other type of calcs.