Hasn't it been stated before by Tony Miller that some of the leathers/treatments have a silica content? I don't want to speak for him obviously, but that could explain a "trace" abrasive on an otherwise untreated strop.

Maybe this also explains why plain leather (no treatment like a latigo finish - Joe?) will not improve sharpness. Is this reasoning for why a proper strop is required, and not your trouser belt?