The presidio is actually a government property, so any crime within the presidio is actually a federal crime and is reported differently. Beware of committing any crime there, the consequences are far more drastic.

Thanks for clearing that up.

That's very likely just a function of population density - this is raw crime density by land area.

EDIT: There's a crime density cutoff below which areas are blank. Within the bounds of SF, blank means very low crime density.