Origin of Eq. 4-11 SMACNA Round Industrial Duct Construction Standards

[fatmax37]

I was wondering if anyone out there knew the origin of equation 4-11 pg. 4.6 in the SMACNA Round Industrial Duct Construction Standards. The section modulus is calculated as follows: Sf=0.25ΩD^2t. This differs from the section modulus for a thin walled pipe that I am familiar with S=Ω(Do^4-Di^4)/32Do. Can anybody explain the reason behind this difference? I contacted SMACNA, but they have not gotten back to me as of yet.