I live about two hours from a major AMZN distribution hub, people I know who live an hour away work there. AMZN is always 5 days.
My hunch is that the criterion for fast shipping is close to “do you have a Cheesecake Factory in your town?”. If you’re in a large city with competitive shopping, AMZN wants to compete, if you live in a place which gets less attention from major retailers, AMZN goes with the herd.
It is no wonder why AMZN has premium service to NYC (Wouldn’t want Jim Cramer to say that AMZN is the slowest shipper) and Washington DC (If your congressman gets fast shipping he thinks his constituents do too) and Los Angeles (otherwise slow shipping from AMZN would be a running gag on all the sitcoms)