Addendum1: I haven't compiled a detailed cross referenced list of observations about HN .. but it's got a lot of little subtle quirks from custom coding .. I suspect there's a window for submission vouching that's only open to users with certain other privlege escalations, or perhaps a stochastic element throws that chance to a random few .. eg: I have no option to vouch for this flagged submission, but I have had that option on others.
Addendum2: Jtsummers may well be right. There may also still be other odd little factors <shrug>.