Crime is one of the problems that causes the disparity.

"Incredibly unclear"? Really? Not the OP, but let's spell it out:

Manicuring a lawn is expensive and serves no useful purpose, so most practitioners have disposable income and/or spare time, and care about keeping up appearances.

Putting a car up on blocks implies it's being used for spare parts, which is generally not something the well-off need to engage in (they buy new and use professionals for repairs as needed), and having one on your front lawn in particular implies that you value spare parts more than appearances.

So both are reasonable proxies of wealth, and wealth correlates inversely with the kind of violent crime you could detect with gunshots.

What seems more obvious to me is that the more surveillance you do in any area (regardless of whether the lawns are manicured), the more crime you're going to find there. If you put 5 cops patrolling the wealthiest, nicest areas of your city, and 1 cop patrolling a less wealthy less nice area, the 5 cops will find more crime than the 1 cop. Even if you don't assume crime is uniformly distributed (which it probably isn't), it's logical that more surveillance -> more crime found.