Working with DoD contracts all day (but not associated with this one so this is strictly a personal opinion)
The original RFP called for a single source to provide pistols, parts, ammunition, suppressors, sight/optics, and holsters. SIG has all of these in house. GLOCK doesn't. I suspect that's your difference right there.
Much like the original Beretta vs SIG in the 1980's, something other than raw test data was the deciding factor.
Aloha