I wasn't referring to the US as a whole, but just instances where I've seen this happen. Most recently it was just in the past few weeks right here on HN that s

I wasn't referring to the US as a whole, but just instances where I've seen this happen. Most recently it was just in the past few weeks right here on HN that someone said crime had risen in Seattle and someone else said it had fallen, and it turned out the discrepancy was due to population adjustment. But I'd expect there's a fair bit of the effect you're saying too.