drywall calculations

[Bob Cook]

I've had a prior version and this 9.54 version for obviously a long time but have used it rarely so my question will be very basic: After completing a structure and calculating materials, I realize I don't know how the program performs it's calculations. As an example, to calculate how much drywall an interior would require, I went back and created an 8' x 8' room (interior dimensions) with 6" x 96" walls and no openings. The materials list tells me I need 2 - 4x8 sheets on the lid with which I agree, but it also indicates that the walls require 11 sheets of material instead of 8. Why is this? Even if the program were to use outside dimensions, only 9 sheets of material would be required. Obviously in an entire structure, the discrepancy would probably be minimal, but I'd still like to understand what I don't know.