Don't forget, there's different kinds of crimes with different visibilities.

So there could be a 50% decrease in violent crime in the most dangerous part of a city, along with a 500% increase in non-violent or petty crimes everywhere else: shoplifting, parked car smash-and-grabs, urinating in the train stations, pooping on the sidewalks, etc. So the tech crowd isn't going to notice fewer people getting murdered in the worst section of town, but they'll definitely see the other stuff.