The expression for volume calculation in G4VTwistedFaceted (geant4.10.07.p02):
fCubicVolume = 2 * fDz * ( ( fDx1 + fDx2 ) * fDy1 + ( fDx3 + fDx4 ) * fDy2 )
The correction expression should be:
fCubicVolume = z*(x1y1+1/2(x1*(y2-y1) + y1*(x2-x1)) + 1/3*(x2-x1)(y2-y1)),
where the parameters are the FULL dimensions (z=2fDZ, x1=2*fDx1, …)
I have reported this directly to the developers and they agree, but I am unable to submit a bug report to https://bugzilla-geant4.kek.jp/, since I am unable to register with an institutional account. Hopefully someone with access to bugzilla can report this.