To add to this quagmire with an alternative theory: Perhaps all applications that received a late February update successfully made it through the first round of reviews and were evaluated by DoD agencies. From here, maybe applicants were a) selected for the fellowship or b) not selected. If selected, maybe the late February application update stuck as the final update. If not selected in a first round of reviews by DoD agencies, it's possible that evaluations were re-evaluated and from there were either selected as fellowship alternates or were rejected.
So in this wacky scenario, a late February update *could* mean that applicants were shortlisted as fellows, while later April updates could indicate that applicants were either selected as fellowship alternates, or, were not selected.
I would place very little weight in any attempts at drawing correlations between NSF GRF awards/Honorable Mention status and association with NDSEG. The application processes for grants and fellowships can be unpredictable, and priorities of NSF and DoD are also quite different.